Kinematics

Kinematics of the knee joint characterizes the relative motion that exists between femur, tibia, and patella. The patella is considered a sesamoid bone implanted within the tendon of the extensor mechanism. Arguably, the most important function of the patella is its role in facilitating extension of the knee by enhancing quadriceps efficiency through increasing the distance of the extensor apparatus from the axis of flexion and extension.¹²⁸ The patella exerts a mechanical advantage by acting as a fulcrum, thus anteriorly displacing the line of pull and increasing the moment arm of the quadriceps muscle force in relation to the center of rotation of the knee. In doing so, the patella may enhance the force of extension by as much as 50% throughout the entire range of motion.²⁸⁵ In addition, the patella allows for the distribution of the reaction force on the femur by increasing the contact area during knee flexion. It also acts as a guide for the extensor mechanism by centralizing the divergent pull from the four muscles of the quadriceps and transmitting these forces to the patellar tendon. Together with the anatomic shape of the patellofemoral articulation, this protects the extensor apparatus from dislocating.

When the knee is extended, the tightening quadriceps pulls the patella upward until the upper border reaches beyond the femoral trochlea groove. So long as the line of gravity falls behind the center axis of the knee joint when standing upright, the quadriceps must contract to neutralize the rotatory effect of gravity on the knee, which would otherwise force it into flexion. As soon as the line of gravity falls within or in front of the knee, which usually occurs in full extension or hyperextension, the quadriceps becomes relaxed. The quadriceps apparatus, being oblique in its angulation toward the patella and patellar tendon, creates a line of pull with an outwardly directed horizontal component when contracted. The angle between the line of pull and patellar tendon is often referred to as the Q angle, which measures on average between 10 and 20 degrees (Fig. 113-1).⁹¹ In coronal and axial planes, this gives rise to a tendency of the patella to slip outward over the lateral femoral condyle, giving a resultant force in the lateral direction.^92,316 To offset this propensity, the lateral condyle projects farther forward, whereas the fibers of vastus medialis that secure the patella medially extend farther distally compared with those of the vastus lateralis.²⁸⁵ In addition, the tibia derotates during the first 30 degrees of flexion, essentially reversing the screw-home mechanism and significantly reducing the Q angle, which in turn also decreases the lateral vector.^{35,92,111,203} Because of a lack of mechanical engagement and the presence of lateral force vectors, the patella is most vulnerable in the early flexion range. Even at 30 degrees of knee flexion, the patella will not have fully engaged into the trochlea groove and the effect of the Q angle may not have neutralized. Walker³¹⁶ has conceded that in the midrange of flexion, the condition for lateral stability of the patella is that the angle of inclination of the lateral trochlear groove is larger than the Q angle.

Figure 113-1 The quadriceps angle (Q angle), a measure of the overall rotational alignment of the lower leg, is affected by the knee flexion angle. As the knee progresses from full extension into flexion the tibia rotates internally, reversing the screw-home mechanism, reducing the Q angle, and assisting in centralizing patellar tracking.

(Adapted from Tria AJ Jr, Klein KS: An illustrated guide to the knee, New York, 1992, Churchill Livingstone.)

Contact Areas

With the knee in full extension, the patella is usually out of contact with the trochlear groove. Depending on the length of the patellar tendon, the patella is drawn into the trochlea from a slight lateral position and gains contact with the femur between 10 and 20 degrees. The contact begins with the inferior margin of the patella and moves proximally as flexion proceeds. Beyond 30 degrees, the patella settles into the deepening trochlear groove, where it is further stabilized by the quadriceps and patellar tendon force. During flexion and extension, the patella presents an excursion of 5 to 7.4 cm in the sagittal plane, whereas the distance between the lower pole of the patella and tibial tuberosity remains constant.^35,93,276

The patellofemoral contact area extends from the medial margin of the medial facet to the lateral margin of the lateral facet as a broad band of contact moving from distal to proximal.^{3,110,112,301} At 60 degrees of flexion, the contact is across the center, at 90 degrees of flexion the contact is toward the superior pole and, beyond 90 degrees, the patella, lying across the medial and lateral condyles, is forming two separate contact areas^131,140 (Figs. 113-2 and 113-3). Ficat and Hungerford⁹² have described this movement of the patella in the coronal plane during flexion as a “gentle curve with its concavity facing laterally.” Studies of the tracking pattern of the natural patella have confirmed that the patella rotates as much as 15 degrees in relation to the femur at flexion angles beyond 50 degrees, tilts in a mediolateral direction in the axial plane, and undergoes medial displacement of up to 4° during early knee flexion, and lateral displacement of up to 8° during high flexion beyond 60°.^{156,247,309,310}

Figure 113-2 Bifurcation of patellofemoral contact area beyond 100 degrees of knee flexion leads to a significant decrease in patellofemoral contact area.

(Adapted from Hehne HJ: Das patellofemoralgelenk [The patellofemoral joint], Stuttgart, Germany, 1983, Enke.)

Figure 113-3 Patellofemoral contact areas at various degrees of knee flexion based on an applied quadriceps force of 1110 N (250 pounds).

(From Walker PS: Human joints and their artificial replacement, Springfield, Ill, 1977, Charles C Thomas.)

In the transverse or axial plane, as seen on skyline radiographs, the patella is perfectly congruent with the trochlea, ensuring its medial and lateral stability. Longitudinal sectioning of the patella has confirmed that the patella adopts almost a flat surface in the sagittal plane, making it perfectly unconstrained as far as its anatomic form is concerned.^142,166 The form of the patella provides stability against lateral subluxation but does not impede it from rocking around its transverse axis to the point at which the resultant of the PFJ reaction force is perpendicular to the contact surface.

As with the location of patellofemoral contact areas, the size of the contact areas is highly dependent on knee position. From 20 to 60 degrees of flexion, the average contact area increases linearly from approximately 150 to 480 mm².^3,302 It then remains almost constant up to approximately 90 degrees of flexion, after which a linear reduction occurs.^112,139 Because of the drastically changed contact pattern beyond 100 degrees, when the patella leaves the trochlea, straddling the intercondylar notch, contact areas may fall well below 100 mm² at full flexion (see Fig. 113-3). Subsequently, only a certain proportion of the retropatellar surface will be in contact with the femur at any one time. Matthews and associates¹⁹⁶ have shown that on average only 19% of the patella-bearing surface is engaged at 30 degrees of flexion, 29% at 60 degrees, 28% at 90 degrees, and 13% at 120 degrees.

Biomechanics

Much attention has been given to define the force transmission in the PFJ. For ease of calculation, it is sufficiently accurate to consider that these forces lie in a sagittal plane. The patellofemoral reaction (or compressive) force (PRF) is acting perpendicular to the articulating surface of the patella and is equal and opposite to the resultant of the patellar tendon and quadriceps forces (Fig. 113-4). With increasing flexion, the line of reaction moves upward, leading to an increase in PRF for the following reasons. As the angle between the patellar tendon and quadriceps becomes more acute, the resultant force vector increases. With knee flexion, effective lever arms of the femur and tibia increase, requiring greater quadriceps power to resist the flexion moment of the body weight (BW). Close to extension, the PRF is only about one third of the quadriceps force, whereas beyond 60 degrees of flexion, the patellar force is about 1.25 times the quadriceps force.³¹⁶

Figure 113-4 The PRF bisects the angle made by the quadriceps and patellar tendon forces and moves upward on the patella with flexion. The TRF occurs through engagement of the quadriceps tendon with the femoral trochlea at 50 to 90 degrees of flexion (turn-round of forces), providing a load-sharing function to the patella. In the coronal and axial views, the sideways component is balanced by the reaction occurring on the slope of the femoral trochlea.

(Adapted and redrawn from Walker PS: Contact areas and load transmission in the knee. In American Academy of Orthopaedic Surgeons: Symposium on reconstructive surgery of the knee, St. Louis, 1978, Mosby, pp 26–36 and Walker PS: Human joints and their artificial replacement, Springfield, Ill, 1977, Charles C Thomas.)

The most important variable in the calculation of static forces is the distance between the line of body weight (center of gravity) and PFJ. Changes of posture in the sagittal plane (leaning forward or backward) will alter this distance and lead to substantial differences in static force transmission, whereas changes in the coronal plane will exert little influence.^16,66 In full extension, the center of gravity usually falls anterior to the knee and moment arms become 0; hence, no forces will be acting on the PFJ. In this situation the body is instinctively balanced by compensatory deviations of its different parts creating a condition of unstable equilibrium. Whenever the line of body weight is moved away from the PFJ in a posterior direction, muscle activity and tension in the patellar and quadriceps tendon increase to maintain position, resulting in higher PRFs.

During normal activities requiring flexion under load, hip flexion is also present, thus bringing the center of gravity forward and shortening the femoral lever arm. This relationship is exemplified by the patient with quadriceps weakness who can rise from a chair by leaning forward, bringing the center of gravity closer to the knee. Similarly, there are significant differences in patellofemoral compressive forces when ascending and descending stairs (Fig. 113-5). Predicted force values for stair ascent range from 1.8 to 2.3 × BW, compared with those for stair descent of 2.9 to 6 × BW^8,223 (Table 113-1). The increased values on descending are a result of the center of gravity being moved further backward behind the PFJ, as the body adopts a more upright position to maintain balance.^17,18,261 Force transmission in the PFJ is therefore dependent on the relationship between the center of gravity of the body and knee flexion angle, and calculations should not be based on the length of the femur and position of the hip joint per se.¹⁶

Figure 113-5 The center of gravity (CG) is positioned almost in line with the patellofemoral joint during stair ascent; thus moment arms of the femur and tibia are relatively short and the PRF is low. During stair descent, the CG is positioned further behind the PFJ, creating longer moment arms and leading to an increase in the PRF.

(From Schindler OS, Scott WN: Basic kinematics and biomechanics of the patello-femoral joint. Part 1: The native patella. Acta Orthop Belg [in press].)

Table 113-1 Patellofemoral Joint Reaction Forces For Various Activities*

The quadriceps tendon abuts onto the proximal aspect of the femoral trochlea in midrange of knee flexion, allowing compressive forces to become divided between the tendofemoral and patellofemoral contact areas.^{18,93,102,214} This phenomenon, described as the “turn-round” of forces, represents an elegant way of maintaining relatively constant unit load under a mechanical situation in which total load is increasing¹¹² (see Fig. 113-4). The efficacy of the turn-round of the divided forces is dependent on the length and altitude of the patella, and it takes effect between 50 and 90 degrees of flexion.¹⁶ According to measurements obtained by Hehne,¹³¹ the contact area of the quadriceps tendon is significantly larger compared with the contact area of the PFJ. At 90 degrees, the quadriceps contact area was one to two times larger, at 120 degrees two to three times larger, and at 140 degrees three to four times larger than the patellofemoral contact area. Huberti and Hayes¹³⁹ have calculated average tendofemoral reaction forces (TRFs) at 120 degrees of flexion as approximately 550 N, whereas PRFs at the same degree of knee flexion measured on average 1600 N, indicating a TRF-to-PRF ratio of 1 : 3. This may partly explain the higher frequency of chondromalacia in patella alta, because tendofemoral contact may be eliminated or substantially decreased, creating an increase in PRF.^148,168

Static measurements of PRFs show an almost linear increase, with values ranging from 0.2 × BW at 10 degrees to 12.9 × BW at 135 degrees.* In vivo PRFs vary and are dependent on the type of activity performed. Predicted force values range from 0.6 × BW for level walking to 7.7 × BW for jogging and 20 × BW for jumping.^{158,214,223,277,329}

Reaction forces during cycling are generally lower than those generated through daily activities and most other athletic activities. Ericson and Nisell⁸⁵ were able to show that the magnitude of joint forces is almost independent of body weight, but increases with workload and reduced seat height. TRF rose to 295 N at 108 degrees of knee flexion, whereas PRF peaked at 83 degrees with 905 N. Anterior knee pain (AKP) during cycling is therefore more likely to occur if the seat position is kept low, because this will increase knee flexion angles throughout all stages of the revolution.⁷³ Cycling should be considered a recommended activity for most patients following knee arthroplasty, especially those who are obese, but proper attention should be given to the level of workload and seat height (see Table 113-1).

When considering the magnitude of PRF, it has to be remembered that this force acts through an area that varies with knee flexion.^190,301,316 Thus, an increase in PRF does not necessarily assume an increase in patellofemoral pressure. The patellofemoral contact area is small near full extension of the knee, indicating that patellofemoral pressure is higher for the same magnitude of PRF. Subsequently, patellofemoral pressure near extension may be relatively high, although the compressive force appears to be comparatively low.²¹⁴

To obtain a rough estimate of the resulting contact pressures in the PFJ, the mean pressure is calculated by dividing patellofemoral force values by the patellofemoral contact area. Calculated patellofemoral contact pressure values, dependent on activity and knee flexion angle, range from 1.28 to 12.6 N/mm².^139,196 A man of 71 kg (696 N) climbing stairs would generate a patellofemoral compressive force of 1754 N equivalent to 2.5 × BW and experience patellofemoral pressures between 3.73 and 6.87 N/mm². Women have smaller knees and hence shorter patellar tendon moment arms than men. Subsequently, the PRF increases by up to 20% for the same knee-extending moment, which would explain the somewhat higher frequency of patellofemoral disorders in women.²¹⁴

Denham and Bishop⁶⁶ have shown that the PRF exceeds the tibiofemoral reaction force in angles above 25 degrees. At almost full knee flexion, these values rise to almost 150% of the forces passing through the tibiofemoral joint. It is therefore not surprising that the cartilaginous cover of the patellofemoral articulation is the thickest in the body and allows the transmission of forces to the subchondral bone in such a way that the pain threshold of the richly innervated bone is not surpassed.⁹² The articulating surfaces are able to adapt to the changing surface contours because of the viscoelastic properties of cartilage, which allow for its deformation under load and subsequent increase in pressure-transmitting area. This process is time-dependent and pressure values will thus be different for short-term and long-term loading. The measured values of patellofemoral pressure should therefore be regarded as reference values only, because pressure-transmitting areas of the PFJ increase with increasing load and duration of loading.^{16,110,131,196,302} This may explain why peak stresses of up to 20 times body weight can be tolerated without causing lasting damage, because they are applied for only very short periods of time, whereas long-term application of such loads would invariably lead to cartilage breakdown.

Dome-Shaped Patella

The great majority of currently available patellar components are of the all-polyethylene dome-shaped type (Fig. 113-9; see also Fig. 113-14). Dome-shaped components usually articulate congruently with the trochlear groove in extension but may be exposed to high stresses and point contact in flexion, when the patella comes into contact with the convexities of the femoral condyles. Some of these problems have been addressed successfully through design adaptations of femoral condylar and trochlear geometries. Extension of the trochlear groove concavity onto the inner portion of the femoral condyles has allowed for an increase of patellofemoral congruency in flexion. In addition, the round-on-round shape of the articulation permits patellar tilt to occur without excessive edge loading, a problem associated with modified dome-shaped and anatomic patellar devices. However, failure of cemented, all-polyethylene dome-shaped patellar components is not uncommon; this has been attributed to the exposure of increase shear stresses in vivo, especially in malpositioned components.^{22,98,137,331}

Figure 113-9 Point contact between dome-shaped patellar implant and relatively nonconforming femoral component at higher flexion.

Modified Dome-Shaped Patella

In an attempt to increase the contact area in flexion, the standard dome patella was modified by adding a concave circumferential extension, allowing it to match the curve of the femoral condyles in the frontal plane more closely (Fig. 113-10). The modified dome-shaped patella, also known as Mexican or sombrero hat, improves the articulation with the convexities of the femoral condyles in the intercondylar notch beyond 90 degrees of flexion by increasing metal to plastic contact. Hsu and Walker¹³⁶ have compared the wear pattern of the all-polyethylene dome and modified dome-shaped patellar components in an experimental study. They found that increasing the conformity in the frontal plane increases the life of the component on a wear simulator by more than 20 times. The authors, however, raise concern that the amount of conformity that is acceptable must be considered in relation to patellar motion path, which is affected by rotation, tilt, and sideways shift.^247,309

Figure 113-10 Modified dome-shaped (Mexican hat, sombrero type) patella. The trochlear groove shape is extended onto the femoral condyles to increase metal to plastic contact at higher flexion angles (Triathlon, Stryker).

Anatomically Shaped Patella

Prostheses with an anatomic surface profile have lateral and medial facets and bear certain theoretical advantages. They maximize the contact area, thereby minimizing contact pressures, and create a more conforming articulation, thus decreasing the risk of subluxation.^43,44,141 A variety of anatomically shaped patella implants have been available over the years, including a mobile-bearing variant (Fig. 113-11; see Fig. 113-8). Unfortunately, however, anatomically shaped patellar prostheses are more sensitive to malpositioning and hence more difficult to implant.¹⁸⁵ Thus, only a few anatomic patella implants remain commercially available, with most being discontinued in favor of more forgiving designs.

Figure 113-11 Anatomically shaped, rotating platform patellar component emulating the asymmetrical shape of the native patella. Both the patella and femoral condyle provide conforming surface configuration (Buechel-Pappas, Endotec, Orlando, Fla).

Cylindrical Patella

The cylindrical or saddle-shaped patellar component occupies a fringe position in TKA and is offered by only a few manufacturers (Fig. 113-12). The initial idea was developed by Freeman et al in the late 1970s^99,100; they attempted to combine a high level of congruency and large contact area throughout the flexion range, with freedom of motion in the coronal plane but without compromising component stability. Because of the design specifics, the patella becomes highly dependent on a close-matching geometry of the femoral component in the sagittal plane, hence requiring a femoral trochlea of single radius.¹⁶⁷ The diameter of the patellar component is reduced to 25 to 30 mm, allowing the implant to be recessed into the patella, similar to the inlay technique (see later). Subsequently, the remaining patellar rim participates in articulating with the femoral component and further contributes to stress dissipation. The patellar implant possesses a central peg with a collar and can be used with or without cement. If left uncemented, the implant retains the ability to self-center and to rotate, as has been observed in revision situations for reasons unrelated to the patella.¹⁴ Although fibrous ingrowth may eventually halt this process, the implant is likely to have adopted favorable alignment by this time.³⁰⁸ Despite its relative rotational constraints, the design concept has provided patellofemoral survival rates of 96% to 98.4% at 10 years.¹⁸⁸ Concerns that preservation of cartilage at the patellar rim may negatively affect pain perception has so far not been confirmed.¹⁸⁸

Figure 113-12 Cylindrical patellar component is a highly congruent design with close-matching patellofemoral geometry in both sagittal and coronal planes (Medial Rotation Knee, Finsbury, Leatherhead, England).

Mobile-Bearing Patella

A different biomechanical concept was conceived with the anatomically shaped, mobile-bearing, metal-backed patella^43,44 (see Fig. 113-11). The design rationale is based on the same principle as that of rotating platform tibial components. The clinical performance record of mobile-bearing patellae has been surprisingly good. Although not generally affected by complications otherwise associated with metal-backed prostheses, failures caused by fracture, wear, and dissociation of the polyethylene element have been described.¹³⁸ Reported survival rates of up to 99.5% at 12 years are attributed to the high conformity and low stresses permitted by the mobile-bearing articulation.¹⁵³ The absence of significant backside wear in mobile patellar bearings has led some clinicians to believe that these devices may not actually rotate in service. It has been speculated that the advantages of mobile-bearing patellae may be their ability to compensate for variations in surgical alignment by rotating into a preferential position after engagement with the femoral component, and simply to remain there.²⁰⁰

Metal-Backed Patellar Components

Metal backing became fashionable in the early 1980s. It has since largely been abandoned, with the exception of some uncemented fixed-bearing designs (e.g., Townley TKO System, Biopro, Port Huron, Minn; Foundation Knee, DJO Surgical, Austin, Tex; Natural Knee II, Zimmer, Warsaw, Ind) and rotating platform anatomic designs (e.g., LCS, DePuy Orthopaedics, Warsaw, Ind; AMC, Corin USA, Tampa, Fla; ROCC, Biomet, Warsaw, Ind; Buechel-Pappas, Endotec, Orlando, Fla). Adding a stiff metal layer to the back surface of the polyethylene patella was thought to improve load transfer and protect the fixation interface.^20,317 To accomplish the addition of metal backing to the patellar component without increasing its overall thickness, substantial reduction in polyethylene thickness is required.²¹ These changes in implant design create an increase in contact stresses because of the rigidity of the metal substrate. Subsequently, polyethylene deformation through cold flow, fracture, and dissociation of the polyethylene layer from the metal backing have been observed in retrieved components^96,180,292 (see Fig. 113-7).

Material Science of Ultra–High-Molecular-Weight Polyethylene in Patellar Implants

Because of the great disparity between moduli and strength of cobalt-chrome alloys on one hand and UHMWPE on the other, wear is primarily observed on the polymeric side of the prosthetic patellofemoral articulation (Fig. 113-15; see Fig. 113-7). Notwithstanding its limitations, UHMWPE has evolved as the material of choice for the patellar component, based on the low-friction principle of Charnley. Mechanical properties of UHMWPE are far from being ideal, with yield strength being affected by the level of molecular weight, degree of cross linking, and sterilization method. Uniaxial yield strength of UHMWPE, which equals the lowest stress at which the material undergoes plastic deformation, is estimated at 21 to 23 MPa (1 MPa = 1 N/mm²), according to manufacturer’s guidelines.^{21,59,248,284} Some manufacturers have expressed concern if such stresses are applied on a regular basis. For machinery applications, repeated maximum contact stresses of no more than 10 MPa have now been recommended, a value identical to the yield strength estimated for articular cartilage.²⁴⁵ These values are based on room temperature, whereas the strength of UHMWP may be reduced by up to 25% at body temperature; therefore, values of 5 MPa may be more appropriate for use in vivo.⁴³

Figure 113-15 Focal area of polyethylene deformation in retrieved patellar component. This is created through point contact of the patellar component with femoral condyles at higher knee flexion angles. It is often seen as part of a wearing-in process in partially congruent patellofemoral articulations.

(Courtesy Professor Wolfgang Plitz, Laboratory of Biomechanics, Ludwig-Maximilian University, Munich, Germany.)

In vitro contact stress analysis has confirmed that all-polyethylene dome-shaped patellar components produce contact pressures from 20 to 30 MPa in extension, increasing to 36 to 100 MPa at 90 to 120 degrees of knee flexion, therefore exceeding the yield strength of UHWMPE by up to 400%.^{145,164,200,332} Anatomically shaped rotating platform patellar components produced significantly lower values, mostly remaining below the yield strength of UHMWP.⁵⁹ Wear simulator studies have further confirmed that congruent patellar components (modified dome-shaped and anatomic) exhibit significantly lower rates of creep and wear than dome-shaped designs, again indicating that conformity is critical to wear resistance and protection against post–yield deformation^134,136 (see Fig. 113-15).

Viscoelastic properties of surface cartilage allow for its deformation under load and subsequent increase in pressure- transmitting area. Because of differences in elastic modulus between cartilage and UHMWPE, the prosthetic patella, however, has limited ability to change its surface contact area through variations in patellofemoral load.^{26,131,136,200} Xu and associates³³² were able to demonstrate the effect of patella resurfacing on contact area and pressure in cadaveric knees. The mean contact area between 30 and 120 degrees of flexion in the nonresurfaced PFJ ranged from 70 to 150 mm², whereas peak patellar contact pressures did not exceed 12 MPa. Once resurfaced, the mean contact area decreased almost 10-fold to 10 to 15 mm², creating a dramatic increase in patellar contact pressure values of 50 to 100 MPa. Steubben and associates²⁸⁷ have carried out biomechanical studies assessing patellar surface contact area, compressive forces, and contact pressure using different prosthetic models.^54,207 They found that patellofemoral contact pressure values at knee flexion angles beyond 45 degrees exceeded polyethylene yield strength in all tested components, with peak measurements of up to 75 N/mm² (75 MPa). Results were not a true function of contact area and contact pressure, signaling the importance of design features in the dissipation of contact forces across the PFJ.

Greenwald and colleagues have measured the distribution of patellofemoral surface stresses by mapping areas above and below the tensile yield strength of polyethylene.^116,287 All implants—whether dome-shaped, modified dome-shaped, or anatomic—demonstrated material yielding over their range of flexion. The results indicated the importance of appreciating the location of the yield areas within a given patellar component, because rim-loaded contact areas above yield are more likely to deform and wear. It may thus be conceivable that polymer integrity is not determined primarily on the size of the contact area, but rather on the extent of the surface within this region, which exceeds yield conditions.

Subsequently, contact stresses above the yield strength do not necessarily lead to catastrophic failure, as demonstrated by the large number of relatively undamaged retrievals. As highest values of contact stress are experienced during flexion, variations in a patient’s activity may not expose the patellar component to large cyclic loads frequently enough to accumulate damage. McNamara and coworkers²⁰⁰ have considered the constraining effect of the surrounding polyethylene as responsible for this phenomenon. Yield in polyethylene is characterized by plastic deformation rather than brittle failure, which explains why nonconforming patellar components are capable of wearing in.²⁶ Such surface adaptation has been demonstrated to produce characteristic facets at the margin of the patellar component, increasing the contact area particularly in flexion when there is least congruency between the patella and condyles¹³⁶ (see Figs. 113-9 and 113-15). Although reduction in contact stresses of 23% to 58% through increased conformity have been reported, contact stress values remain above the UHMWP yield strength.^59,127 Elbert and associates⁷⁹ were surprised that despite reduction in overall component thickness, stress values did not increase. In their opinion, changes in geometry and contact area dominate the response of the deformed component, overriding the effect on the stresses of the decreased thickness. Analysis of Von Mises stress, a yielding criterion based on a combination of principle stresses occurring within the material, has revealed that most stresses above yield strength occur 1 to 2 mm below the articulating surface area in the newly manufactured component; however, in retrieved components, Von Mises stress remains near yielding through the depth of the implant.^207,328 Because of subsurface stresses, permanent deformation may be expected to continue, even when the component has worn in. Collier and associates⁵⁹ have noted that all-polyethylene patellar components do not solve this problem because of distinct biomedical and biomechanical disadvantages of UHMWPE as a bearing surface. In the absence of a suitable alternative however, UHMWPE is here to stay, at least for the foreseeable future.

Femoral Component Design

The patella, whether native or prosthetic, cannot be considered separately because it works in direct partnership with the femoral component. Contact areas are highly dependent on the congruency of the PFJ articulation at all angles of knee flexion, whereas motion constraints of the patella are terminated by the surface geometry of the femoral component and by the balance of soft tissue forces. Bartel and associates²⁰ have demonstrated the importance of conformity in prosthetic design to increase contact area and decrease contact stress. Current femoral prostheses display a wide variation in design features in regard to length, depth and orientation of the trochlear groove, sagittal radius, and axial geometry.^75,297,334 Anatomically shaped femoral components appear to be particularly suitable if articulating against the nonresurfaced patella, which will be referred to as patella-friendly from this point on^161,296 (Fig. 113-16). They provide for increased conformity between the native patella and femur and demand minimal biologic patellar remodeling.^43,161,325 Nonanatomic designs are those in which the trochlear groove is concave spherical and designed to accommodate a nonanatomic patella, usually of dome-shaped design. Proximal extension of the femoral flange will help capture the patella during early flexion, whereas extension of the concave shape of the trochlear groove onto the inner aspects of the femoral condyles will allow for increased metal to plastic contact at higher flexion angles (see Figs. 113-10 and 113-14).^{54,56,227,264,297}

Figure 113-16 Various levels of ‘patella-friendly’ design features in six different femoral components. Relatively patella-friendly designs (top row) with asymmetrical, anatomic femoral groove, elevated lateral trochlear flange, and distal extension of trochlear groove (modified Ortholoc, Wright Medical, Arlington, Tenn; Buechel-Pappas, Endotec; Journey PS, Smith & Nephew, Memphis, Tenn). Relatively patella-unfriendly designs (bottom row) with symmetrical, shallow, and short trochlear groove (left to right: Ortholoc, Dow Corning Wright; AGC, Biomet, Warsaw, Ind; Townley TKO, Biopro, Port Huron, Mich).

(Ortholoc femoral components courtesy Leo Whiteside, Missouri Bone & Joint Research Foundation, St Louis.)

The effect of valgus alignment of the trochlear groove compared with symmetrical designs on shear stresses has been investigated with mixed results. Asymmetrical trochlear groove designs are thought to provide earlier patellar capture through prominence of the lateral flange and to decrease the predominant valgus force vector, thus reducing patella shear.⁷⁵ In some reports, reduction in lateral shear forces of up to 10% were observed, whereas others saw no effect or even a shift toward the generation of medial shear forces.^56,227,297 The exact clinical advantages of asymmetrical designs have remained largely theoretical, with no compelling clinical proof of their effectiveness.³¹⁸

The importance of femoral component design and its influence on patellofemoral performance have been highlighted by Theiss and associates²⁹⁷ based on the clinical results of two arthroplasty designs with distinct differences in trochlear geometry. A 14-fold decrease in patella-related complications was observed when using a patella-friendly design with an extended anterior flange and a deeper and wider trochlear groove. It was concluded that the more proximal capture of the patella in a deeper groove, with more gradual proximal to distal transition, appears advantageous in reducing patella morbidity. Yoshii and associates³³⁴ used an experimental model and demonstrated that specific femoral design changes, including deepening and distal extension of the trochlear groove, improve patella tracking compared with an unmodified femoral component.

Lateral forces acting at the patellofemoral articulation increase with knee flexion¹⁴ (see Fig. 113-4). The magnitude of the lateral forces, which are dependent on valgus alignment, Q angle, and soft tissue balance may, if excessive, cause patellar subluxation and contribute to component failure. Steubben and associates²⁸⁷ have investigated the resistance offered to lateral subluxation of the resurfaced patella by defining the intrinsic lateral stability of various patellofemoral designs. They found that the forces required to cause lateral displacement were at or above those measured for the native knee, and highly dependent on the interaction of condylar and patellar surface geometry. Their results highlighted that appropriate design changes can significantly increase resistance to patellar subluxation.

Unresurfaced Patella

Following bicompartmental knee arthroplasty, the nonresurfaced patella becomes exposed to the metallic surface of the femoral component. Because of differences in the modulus of elasticity, the articular surface of the patella must adapt to the geometry of the opposing surface by bedding in.¹⁶¹ This process of remodeling, which has been termed stress contouring, produces a gradual adaptation of the retropatellar surface and subchondral bone plate to the trochlear shape.²⁸⁰ Keblish and associates¹⁶¹ have noted that minimal remodeling is required if the patella is exposed to an anatomic design with a constant radius of curvature and uniform femoral geometry, whereas excessive remodeling is observed in nonanatomic designs (Fig. 113-17). The remodeling process is time-dependent and not seen on axial radiographs much before 2 years after implantation.

Figure 113-17 Skyline radiographs showing an anatomically shaped “patella-friendly” femoral component design (left, LCS [DePuy Orthopaedics, Warsaw, Ind]) compared with a nonanatomic design (right, Optetrak [Exactech, Gainsville, Fla]).

Tanzer and associates²⁹⁶ have studied the effect of femoral component designs on the contact and tracking characteristics of the unresurfaced patella in TKA. They noted substantial alterations in patellofemoral contact areas, contact pressures, and tracking at higher flexion angles when the native patella was articulating with a prosthetic femoral component. Although the percentage of patellofemoral contact area compared with the native knee is markedly reduced with increasing knee flexion, with measured values of 79% at 60 degrees, 69% at 90 degrees, and 65% at 105 degrees, it remains well above those measured for the prosthetic patella.

The surface geometries of some prosthetic femoral components, particularly those of posterior-stabilized design, appear incompatible with the native patella, because the apex of the retropatellar ridge may impinge on the prosthetic intercondylar notch beyond 90 degrees of knee flexion. Patellar deformation and wear are likely consequences and, in the case of significant patellar tilt, displacement of the patella into the notch becomes possible.¹⁹⁸ Distal extension of the trochlea and shortening of the intercondylar notch have been shown to safeguard patellar support beyond 90 degrees of knee flexion.³³⁴ Such design modifications are therefore important if one considers leaving the patella unresurfaced. Most current femoral components present a surface geometry designed to articulate with a designated patellar component but are ill equipped to accommodate the native patella. Specific efforts are required to improve patellar kinematics by creating a femoral component that conforms to the normal trochlear and intercondylar notch topography, and that takes the geometry of the native patella into account.³²⁵ Only then would we be in a position to offer prostheses dedicated to be used with the native patella, compared with the mostly inadequate femoral designs currently available.

Effect of Cruciate Retention or Substitution

Moment arms affecting the patellar are dependent on the distance between the PFJ and the axis of flexion and extension of the femoral component. They are increased if the axis is deviated posteriorly from its physiologic position. Femoral rollback facilitates this process and represents a characteristic feature of normal knee kinematics. Increased rollback effectively lengthens the patellar moment arm thus increasing the efficacy of the extensor mechanism. D’Lima and associates⁷⁵ have investigated the influence of various degrees of posterior femoral rollback on patellofemoral compressive forces. Femoral rollback resulting from PCL preservation produced reductions in patellofemoral compressive forces of up to 7% throughout knee flexion, whereas the effect in PCL-substituting devices only became noticeable after cam-post engagement, with a maximum effect recorded at 85 degrees of knee flexion. Miller and colleagues,²⁰⁵ in an earlier study comparing PCL-retaining with PCL-substituting arthroplasties, failed to note femoral rollback when the PCL was retained. They stipulated that the absence of the anterior cruciate ligament may render the PCL ineffective, which may explain the appearance of paradoxical movements (reverse rollback)⁷⁵ observed on fluoroscopic investigation.^68–70 Although PCL substitution kept patellofemoral forces close to the level of the native knee, a lateral release became necessary in 50% of knees, raising potential concerns about an increase in patellofemoral stress through ligamentous tension. This concept has also been expressed by Ranawat and Sculco^233,235 who raised concern that femoral rollback, through a cam and post mechanism, as in posterior-stabilizing designs, or through a functional posterior cruciate ligament (PCL), may increase tensile forces across the patella in flexion. Overstuffing will tighten the extensor mechanism, and increase anterior patellar strain and PRF, with the likely consequences of loss of flexion, patellar implant failure, and fracture.^{78,204,235,303} Overall patellar thickness following resurfacing should therefore not exceed preoperative values. With regard to clinical outcomes, Waters and Bentley³²⁰ have assessed the occurrence of postoperative anterior knee pain in 327 cruciate-substituting and 147 cruciate-retaining TKAs, but found no significant difference.

Femoral Positioning

The patella is particularly sensitive to malrotation of the prosthetic implants of the femur and tibia.^10,107 It is generally accepted that placing the femoral component in approximately 3 to 5 degrees of external rotation, relative to the posterior condylar axis, improves patellar tracking and proximal engagement.²¹² External rotation moves the trochlear groove toward the natural position of the patella, thereby relaxing the lateral retinacular structures and reducing the lateral force vector acting on the patella. Second, it also corrects any mismatch in the flexion-extension gap created through cutting the proximal tibia perpendicularly to its long axis, rather than emulating its natural varus alignment. This can be achieved through the relative thickness of bone removed from the posterior femoral condyles, but care must be taken to establish a rectangular flexion gap; otherwise, knee stability in flexion may be compromised. Any instrumentation for distal femoral preparation that references femoral component rotation on the posterior condyles would, if left unadjusted, place the femur in internal rotation relative to the transepicondylar axis. Posterior condylar referencing therefore requires the surgeon to dial in a degree of external rotation to achieve parallel cuts to the transepicondylar axis (TEA), which has created what is referred to as external femoral rotation. Whiteside and coworkers²¹¹ have found the patellar tracking pattern and contact areas to be most normal when the femoral implant is externally rotated by approximately 5 degrees in relation to the PCA.¹⁰ External rotation angles beyond 5 degrees, however, appear to have a detrimental effect on the spatial position of the patella in the trochlea.¹³ Because of the variability in the relationship between the posterior condylar axis (PCA) and TEA, this technique carries certain dangers. Although the main difference between the two axes is 3 degrees of internal rotation, the range is 0 to 10 degrees, with higher values for valgus knees.²¹⁹ The risk of femoral malrotation is thus higher in the valgus knee, especially if a somewhat hypoplastic posterior lateral femoral condyle has been used as a reference for rotational alignment when making anteroposterior cuts. The situation is usually compounded because the internally rotated femur necessitates subsequent internal rotation of the tibial component to establish rotational tibiofemoral congruency, thus creating excessive external rotation of the tibial tuberosity and patellar maltracking.

Because the TEA represents the functional flexion-extension axis of the knee, it has been established as the ideal reference point for femoral component orientation.^24,144,230 Studies have confirmed improved patellar kinematics and a significant reduction of patellofemoral shear forces when the femoral component is placed parallel to the TEA as compared with internal or external rotation.^75,205

As an alternative reference to the TEA, the anteroposterior axis, or Whiteside’s line (WL), drawn from the base of the trochlear groove to the center of the intercondylar notch, may be used to establish femoral rotational alignment.¹² Current technique encourages clinicians to use TEA and WL to place the femoral component in neutral rotation with respect to those landmarks (Fig. 113-18).

Figure 113-18 The TEA and the anteroposterior axis (WL), are used as reference guides for femoral component rotation and have shown to be more reliable than the PCA.

(Adapted from Krackow KA: The technique of total knee arthroplasty. St Louis, 1990, Mosby.)

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