The femur is the longest and strongest bone in the body. Its shaft is nearly cylindrical and fairly uniform in caliber; however, an extremely variable bow can often be present. The caliber is important when using intramedullary referencing for femoral arthroplasty, particularly when stem fixation of the femoral component is desired. Marker balls used on the preoperative radiographs are recommended in cases in which the canal diameter is needed. Bowing, when excessive, can affect the radiographic measurements used in TKA. Long-cassette anteroposterior and lateral radiographs are most accurate in assessing the femur when concerns are present.²,³

The distal aspect of the femur broadens approximately threefold into the medial and lateral condyles. All but the sides of these condyles are articular. The inferior, posterior oblong-shaped portions of the condyles articulate smoothly with the tibial plateau, whereas the central, anterior surface between the condyles articulates with the
facets of the patella. The inferior, oblong surfaces of the condyles are separated by the intercondylar fossa, which houses the cruciate ligaments. These ligaments can be damaged and tension altered by osteophyte formation in patients with osteoarthritis.⁴ The intercondylar fossa is especially deep posteriorly and is separated by a ridge from the popliteal surface of the femur superiorly.⁵ This surface, bounded by the two supracondylar ridges where the gastrocnemius muscles originate, is in contact with the popliteal artery. The bone in the deep intracondylar fossa is hard and unyielding. Therefore, if cutting this posterior bone for the box resection to house a cam-post mechanism of a posterior-stabilized knee, the correct depth is critical to prevent condylar fracture during implant impaction. The lateral condyle has a greater posterior excursion than the medial so that internal rotation of the femoral cuts during TKA seldom results in notching of the posterior femur except in cases of excessive component downsizing or rotation. Because there is little deformation of the posterior condyles in the arthritic varus knee, a posterior condylar line can be used as a guide to estimate femoral implant rotation, as will be discussed later.⁶ The medial condylar surface is longer than that of the lateral and is flatter anteriorly and more curved posteriorly. In the sagittal plane, the radius of curvature of both condyles is greatest posteriorly as mentioned, but the larger medial condyle is slightly more symmetric.⁷ Rotation can affect the apparent size of the condyles on radiographs.⁸

The contact surface for the patella is derived mostly from the lateral condyle. The anterior extensions (rises) of both condyles form a fossa for the patella to sit in extension. The lateral rise is the greatest. Therefore, with external rotation of the femoral cut (to improve patellar tracking in TKA), more lateral bone is taken from the anterior femur, increasing the likelihood of notching laterally. The lateral condyle is broader than the medial condyle, and in the frontal plane, the lateral condyle is slightly shorter than the medial, giving rise to the valgus angulation of the distal femur (this is why the distal femoral cutting guide will typically rest against the medial femoral condyle and not the lateral). With weight-bearing, however, the two condyles rest on the horizontal plane of the tibial condyles, and the shaft of the femur inclines downward and inward. This inclination is the result of the greater breadth of the body at the pelvis than at the knees.

The epicondyles of the femur bulge above and within the curvature of the condyles. The medial epicondyle is more prominent and provides attachment for the tibial collateral ligament and also remains the insertion point for the adductor magnus muscle at the adductor tubercle. Behind this tubercle, the bony surface is roughened and provides the origin of the medial head of the gastrocnemius muscle. The lateral epicondyle gives rise to the fibular collateral ligament and the lateral head of the gastrocnemius. In addition, the plantaris muscle arises posteriorly. Immediately below the lateral epicondyle and bordering the articular surface of the condyle is an oblique groove that houses the tendon of the popliteus muscle.

Berger et al. found that the surgical epicondylar axis—defined as the line connecting the lateral epicondylar prominence and the medial sulcus of the medial epicondyle—was a useful reference for the rotational orientation of the femoral component during primary or revision TKA when the posterior condylar surfaces could not be used.⁹ Similarly, using magnetic resonance imaging (MRI), the transepicondylar axis was found to be a reliable rotational landmark⁶ and is approximately 6° externally rotated relative to the posterior condyles in both normal and varus knees (with no medial condylar posterior bone loss). More recently, Miller et al. found that femoral component rotation parallel to the surgical epicondylar axis maximized patellar tracking within the femoral sulcus and minimized tibiofemoral wear motion and that these benefits were less reproducible when using the posterior condylar axis as a reference.¹⁰

Femoral bony landmarks can also be used to estimate the joint line, an important consideration in primary and revision knee surgery. Absolute distances have been previously published; however, these have been shown to vary depending on bone size and gender. Therefore, more recent studies have focused on anatomic ratios. Servien et al. found that the normalized ratios between the distance from the medial and lateral epicondyle to the respective distal condyle articular surface and the femoral width were 0.34 and 0.28, respectively.¹¹ Importantly, these values did not vary with gender. Re-creating an anatomic joint line is important in both primary and revision surgery as alteration can potentially lead to patella baja/alta, flexion-extension mismatch, and midflexion instability.

The proximal tibia is expanded to receive the condyles of the femur. The shaft of the bone flares out into lateral and medial buttresses, which form the medial and lateral condyles. The tibia is the weight-bearing bone of the leg, whereas the fibula serves for muscular attachments and for completion of the ankle joint. The superior articular surface of the tibia presents two facets. The medial facet is oval in shape and has a slight concavity. The lateral facet is nearly round, and although concave from side to side, it is convex in front. The rims of these facets are in contact with the medial and lateral menisci, but the central portions receive the condyles of the femur. A femorotibial offset exists, with the center of the femur being medial and anterior to the center of the tibia.¹² If this offset is present preoperatively, a posterior-stabilized knee design with a cam-post mechanism may be the prudent option to resist this intrinsic deformity and to prevent it from occurring postoperatively. An intercondylar eminence
with two tubercles arises between the two articular facets. The articular surfaces continue on to the adjacent sides of the medial and lateral intercondylar tubercles. Anterior to the intercondylar eminence is the anterior intercondylar area, which provides attachment for the anterior horns of the medial and lateral menisci and the anterior cruciate ligament (ACL). The posterior intercondylar area is a broad groove that separates the posterior aspects of the condyles. The posterior cruciate ligament (PCL) originates in the posterior intercondylar area approximately 1 cm inferior to the joint line and a few millimeters lateral to the center of the lateral intercondylar tubercle.¹³ From its origin, the PCL travels anterior and slightly medial, where it is joined by a cord from the lateral meniscus (the posterior meniscofemoral ligament, or ligament of Wrisberg) to attach to the medial condyle of the femur. When exposing the tibia during a cruciate-retaining TKA, care must be taken not to sever the PCL. In addition, tibial cuts that are excessively deep or posteriorly sloped may violate the origin of the PCL. The posterior intercondylar area also gives attachment to the posterior horns of the medial and lateral menisci. Anteriorly, the two condylar surfaces blend in a triangular area that leads distally to the tibial tuberosity. This anterior tibial triangle has large vascular foramina and is sharply marked at its borders by oblique lines where the fascia lata attaches. The tibial tuberosity is smooth superiorly and rough inferiorly for the termination of the patellar ligament. The proximal aspect of the rough area identifies the safe level of resection that will not harm the patellar tendon insertion. Posteriorly, the medial condyle is marked by a transverse groove that accommodates the insertion of the tendon of the semimembranosus muscle. The rough medial surface of the condyle gives attachment to the tibial collateral ligament. The lateral condyle has a nearly circular facet on its posteroinferior surface for articulation with the head of the fibula. Anterior to this facet, at the junction of the anterior and lateral surfaces of the condyle, is an oblique line on which the iliotibial band (ITB) attaches.

The body of the tibia is expanded at its extremities but is otherwise fairly uniform in size. In cross section, it is triangular, with medial, lateral, and posterior surfaces and having anterior, medial, and interosseous borders. The anterior border (crest), just distal to the tibial tubercle, is subcutaneous and prominent. It is slightly sinuous, beginning at the lateral margin of the tuberosity proximally and moving medially distally. Therefore, the midline of the tibia for extramedullary alignment during TKA is found just distal to the tubercle, where the crest has moved medially. The intramedullary canal relative to the proximal tibial plateau is such that a rod placed down the tibia will exit slightly medially within the plateau. At its upper medial portion, the tibia provides for the attachment of the tibial collateral ligament and the more medial fibers of the popliteus muscle. The soleus muscle arises from the middle one-third of the medial border. The interosseous border is on the fibular side of the tibia and is sharp throughout its length. Superiorly, it begins below and anterior to the facet for the head of the fibula and provides, throughout its length, for the attachment of the interosseous membrane.

The medial surface of the body of the tibia is smooth and convex. Its upper one-third receives the insertions of the sartorius, gracilis, and semitendinosus muscles. The distal two-thirds of the medial surface are subcutaneous. Gerdy tubercle is the proximal, most lateral prominence. The fascia lata can refer pain from the hip to this region in cases of severe trochanteric bursitis. The lateral surface has a shallow groove in its upper two-thirds, which provides the origin of the tibialis anterior muscle. The lateral parapatellar approach in TKA usually requires release to this point during lateral exposure and anterior subluxation of the patella.

The most prominent marking on the posterior surface of the tibia is the soleal line. Beginning behind the facet for the head of the fibula, this line extends obliquely downward across the back of the tibia. The triangular area above this line gives insertion to the popliteus muscle. The soleal line itself serves for the attachment of the popliteus fascia and as origin for the soleus muscle. Occasionally, release along this line is necessary to increase the medial flexion gap.

The tibia slopes posteriorly, but with great variability, generally angling in a range of 5° to 10° from the intramedullary canal. The bone trabeculae are aligned relatively perpendicular to this slope orientation but are greatly affected by osteoarthritic changes. Studies have demonstrated the variables affecting bone strength after arthroplasty.¹⁴,¹⁵ Posterior angulation matching the anatomic angulation is likely to optimize the strength of the underlying bone.¹⁶ However, when a deep tibial cut is made at a less than anatomic angulation, more distal fixation should be considered to meet the load-bearing needs of TKA.

Recent hypotheses and studies suggest that knee kinematics is described as two simultaneous rotations occurring about fixed axes. Knee flexion and extension occur about an optimal flexion axis in the femur, whereas tibial internal and external rotations occur about a longitudinal rotation axis in the tibia. Churchill et al. found that the transepicondylar axis closely approximated the optimal flexion axis.¹⁷ According to Matsumoto et al., the axis location for tibial rotation remained approximately in the area between the two cruciate ligament insertions throughout the range of flexion. However, the axis did change with changes in cruciate ligament tension and surrounding soft tissues.¹⁸

Similar to the femur discussed above, proximal tibia bony landmarks can be used to estimate the joint line. Using MRI, Servien et al. determined that the tibial tubercle was a reliable landmark for reference, especially when using anatomic ratios. Specifically, the ratio of the
distance between the tibial tubercle and the joint line to the anteroposterior width of the tibia at the level of the tubercle was 0.50 in both males and females. Similarly, femoral width can also be used, as the ratio of the distance between the tubercle and the anterior tibial joint level to femoral width was 0.27.¹¹

The fibula is a long, slender bone that lies parallel and lateral to the tibia. It does not participate in weight-bearing, but rather serves for muscle and tendon attachment and provides stability at the ankle distally. The head of the fibula is knob-like, and superiorly, slanted toward the tibia, is the almost circular articular surface, which participates in the tibiofibular articulation. Violation of this joint during TKA is usually noted by the appearance of articular cartilage in the posterior lateral corner of the resected tibial surface and occurs most frequently with an excessively deep tibial resection. At the posterolateral limit of the articular facet, the apex of the head projects upward and provides attachment to the fibular collateral ligament of the knee joint. The tendon of the biceps femoris muscle attaches to the lateral aspect of the head of the fibula. The circumference of the head is rough and has anterior and posterior tubercles, which provide attachment for the proximal-most fibers of the peroneus longus muscle and upper fibers of the soleus muscle, respectively.

The joint between the circular facet on the head of the fibula and a similarly shaped surface on the posterolateral aspect of the underside of the lateral condyle of the tibia can be nearly flat or slightly grooved and may be transverse or oblique. Movement is slight at this joint. The articular capsule is attached at the margins of the facets on the tibia and the fibula and is strengthened by accessory ligaments anteriorly and posteriorly. The anterior ligament of the head of the fibula consists of fibrous bands that pass obliquely from the front of the head of the fibula to the front of the lateral condyle of the tibia. The posterior ligament is a single broad band that runs obliquely between the head and the back of the lateral tibial condyle. The tendon of the popliteus muscle crosses it. The subpopliteal recess of the synovial cavity of the knee joint occasionally communicates here with the cavity of the tibiofibular articulation. The joint receives its arterial supply from the lateral inferior genicular and anterior recurrent tibial arteries. Nerves to the articulation are derived from the common peroneal nerve, the nerve to the popliteus muscle, and the anterior tibial recurrent nerve.

The interosseous membrane of the leg extends between the interosseous borders of the tibia and the fibula and consists largely of fibers that pass from the tibia laterally and inferiorly toward the fibula. The upper margin of the membrane does not reach the tibiofibular articulation, and the anterior tibial vessels pass over the upper edge of the membrane to the anterior compartment of the leg.

The patella is a largest sesamoid bone in the body, developed in the tendon of the quadriceps femoris muscle. It articulates against the anterior articular surface of the distal femur. It holds the patellar tendon off the distal femur, thus improving the angle of approach of the tendon to its distal insertion on the tibial tuberosity, increasing the moment arm of the force vector generated by the quadriceps muscle. The anterior surface of the patella is convex and is vertically striated by the tendon fibers. The superior border is thick and gives attachment to the tendinous fibers of the rectus femoris and vastus intermedius muscles. The lateral and medial borders are thinner and receive the tendinous fibers of the vastus lateralis and vastus medialis muscles, respectively. These two borders converge inferiorly to the pointed lower pole of the patella, which gives attachment to the patellar ligament. During arthroplasty, a deep patellar cut may violate this inferior pole. Although a deep cut may yield a larger patellar surface for arthroplasty, it does so to the detriment of patellar thickness and strength. On the other hand, it may allow for slightly inferior positioning of the patella button, helpful in decreasing component loading at higher flexion angles.¹⁹ Also, if the patella is thinned to 10 mm, the lower pole contributes to the overall fixation surface, thus creating a larger, more circular shape for fixation of a larger size button. Thus, this cut must be carefully performed to optimize strength and surface diameter.

The articular surface of the patella has a smooth, oval shape that is greater in the medial to lateral direction. This shape allows for medialization of the patellar component if sized to the smaller superoinferior dimension. The surface is divided into two facets by a vertical ridge that occupies the patellar groove on the articular surface of the femur. The lateral facet is broader and deeper than the medial facet. These patellar facets correspond to convex areas on the patellar groove of the femur. The anterior femoral sulcus has medial and lateral ridges, with the lateral being the highest.

The menisci are crescent-shaped wedges of fibrocartilage that rest on the peripheral aspects of the articular surfaces of the proximal tibia. They function to effectively deepen the medial and lateral tibia fossae for articulation with the condyles of the femur. They are thickest at their external margins and taper to thin, unattached edges as they progress inwardly. The menisci are attached along the outer edges of the condyles of the tibia and behind its intercondylar eminence (Fig. 2-1B). The superior surfaces are slightly concave to accommodate the condyles of the femur, thus providing greater surface area contact. The medial meniscus is larger than the lateral and more ovoid in shape. Anteriorly, it is thin and pointed at its attachment in the anterior intercondylar area of the tibia directly in front of the ACL. Posteriorly, it is broadest, attaching in the corresponding posterior fossa, anterior to the origin of the PCL. The lateral meniscus is smaller and more circular, covering a greater proportion of the tibial surface than does the medial meniscus. Its anterior horn is attached in the anterior intercondylar area, posterior and lateral to the insertion of the ACL. Posteriorly, it terminates in the posterior intercondylar area, anterior to the termination of the medial meniscus. The lateral meniscus is weakly attached around the margin of the lateral condyle of the tibia. In addition, it lacks attachment where it is crossed and notched by the popliteus tendon. This “bare area” is easily identified during TKA and provides a reference for the surgeon as to the safe depth of lateral meniscal excision, thus helping to prevent injury to the lateral inferior geniculate artery during excision of the anterior margin. Near its posterior attachment, the lateral meniscus frequently sends off a collection of fibers, the posterior meniscofemoral ligament (ligament of Wrisberg), which either joins or lies behind the PCL. This ligament ends in the medial condyle of the femur, immediately behind the attachment of the PCL. Occasionally, an anterior meniscofemoral ligament is also present, with a similar but anterior relationship to the PCL. The lateral meniscus is thus loosely attached to the tibia and has frequent attachment to the femur. Therefore, it tends to translate with the lateral femoral condyle during flexion of the knee.
In contrast, the medial mensicus is less mobile with capsular attachments to the tibia (coronary ligament) and femur (meniscofemoral ligament) and is intimate with the deep portion of the medial collateral ligament (MCL) at its periphery. Therefore, when excising the medial meniscus during arthroplasty procedures, care must be taken not to disrupt the MCL at the medial periphery. Finally, the convex, anterior margin of the lateral meniscus is connected to the anterior horn of the medial meniscus by the transverse genicular ligament.

The cruciate ligaments are strong, rounded cords that lie within the capsule of the knee joint and cross each other like the limbs of an X (Fig. 2-1A). They are named anterior and posterior based on their relationships to the intercondylar eminence of the proximal tibia (Fig. 2-1B). The ACL arises from a rough, nonarticular area anterior to the intercondylar eminence of the tibia and extends upward and backward to the posteromedial aspect of the lateral femoral condyle (Fig. 2-2A). In extension, the posterolateral bulk is taut, whereas in flexion, the anteromedial band is tight and the posterolateral bulk is relatively relaxed.²⁰ The PCL arises from the area posterior to the tibial eminence and travels upward and forward along the medial border of the ACL to attach to the lateral aspect of the medial condyle of the femur (Figs. 2-1B and 2-2B). In flexion, the bulk of the PCL tightens, and in extension, it is relaxed (Fig. 2-2B). The ACL prevents anterior displacement of the tibia, and the PCL restricts posterior displacement.

The articular cavity of the knee is the largest joint space of the body. The cavity includes the space between and around the tibial and femoral condyles but also extends proximally, behind the patella, to include the femoropatellar articulation, and further into the suprapatellar bursa, which lies between the tendon of the quadriceps femoris muscle and the femur, where it communicates freely. The synovial membrane lines the articular capsule and reflects onto the bones as far as the edges of their articular surfaces. It also follows the suprapatellar bursa and extends to the sides of the patella under the aponeurosis of the vastus muscles. The synovial membrane covers the recesses that lie behind the posterior areas of each femoral condyle. At the superior-most portion of the medial recess, the bursa under the medial head of the gastrocnemius muscle occasionally opens into the joint cavity. In the subpopliteal recess, the cavity and the synovium lining extend beyond the capsule to lie against the tendon of the popliteus muscle. The synovial membrane also covers the cruciate ligament except where the PCL is attached to the back of the capsule. Thus, the cruciate ligaments are intra-articular structures that lie outside the capsule. The infrapatellar fat pad, which lies below the patella, represents an anterior section of the median septum of tissues that, with the cruciate ligaments, separate the two tibiofemoral articulations. The fat pad is often taut after eversion of the patella during a medial parapatellar approach in TKA. Release of the infrapatellar synovium to the level of the lateral meniscus
can significantly decrease tension when the patella is everted. From the synovial surface of the infrapatellar fat pad, a vertical fold frequently passes toward the cruciate ligaments and attaches to the intracondylar fossa of the femur anterior to the ACL and lateral to the PCL. From the medial and lateral borders of the articular surface of the patella, the synovial membrane projects into the interior joint and curls around to attach adjacent to the cartilage of the medial and lateral femoral condyles.

Because almost all tendons at the knee lie parallel to the bones and pull lengthwise across the joint, bursae are numerous (Fig. 2-3). The suprapatellar bursa lies between the quadriceps tendon and the anterior femur. Three other bursae are associated with the patella and its ligament. The prepatellar bursa, located between the skin and the anterior surface of the patella, allows free movement of the skin over the patella during flexion and extension. The subcutaneous infrapatellar bursa lies between the patellar tendon and the overlying skin. The prepatellar and subcutaneous infrapatellar bursae may become inflamed as a result of direct trauma to the front of the knee or through activities like repetitive or prolonged kneeling. The deep infrapatellar bursa, located between the patellar ligament and the tibial tuberosity, is separated from the synovial cavity of the joint by the infrapatellar fat pad and helps to reduce friction between the patellar ligament and the tibial tuberosity.

Lateral to the joint, the inferior subtendinous bursa of the biceps femoris muscle lies between the tendon of this muscle and the fibular collateral ligament. The subpopliteal bursa (recess of the synovial membrane) lies between the tendon of the popliteus muscle and the lateral femoral condyle. Another bursa may separate the popliteus tendon from the fibular collateral ligament, or the membrane of the subpopliteal recess may wrap around the tendon to separate them. Also belonging to this lateral group is the subtendinous bursa of the lateral head of the gastrocnemius muscle, which lies beneath the tendon of origin of this muscle and occasionally communicates with the knee joint.

Medially, the bursa anserina lies deep to the pes anserinus tendons (sartorius, gracilis, and semitendinosus muscles) and separates them from the tibial collateral ligament. The bursa of the semimembranosus muscle lies between the muscle and the tibia. The subtendinous bursa of the medial head of the gastrocnemius muscle underlies the tendon of the origin of the medial head, separating it from the femur. With the knee flexed, the gastrocnemio-semimembranosus bursa communicates with the knee joint, and this communication closes with knee extension.²¹ Posteriorly, there are two large bursae associated with the medial and lateral heads of the gastrocnemius muscle. The bursa of the lateral head of the gastrocnemius separates the muscle from the joint capsule and occasionally communicates with the knee joint. The bursa of the medial head of the gastrocnemius underlies the medial head, separating it from the joint capsule, and generally communicates with the knee joint. This bursa is the most common site of occurrence of a Baker cyst (in patients with rheumatoid arthritis) or for the collection of substantial debris and synovial fluid extravasation around the failed TKA with osteolysis.

The articular capsule of the knee joint is inseparable from the ligaments and aponeuroses apposed to it. Posteriorly, its vertical fibers arise from the femoral and tibial condyles and the intercondylar fossa of the femur and are covered by the oblique popliteal ligament (Fig. 2-4). Inferiorly, the capsule attaches to the tibial condyles and the borders of the menisci. External ligaments that reinforce the capsule are the fascia lata and the iliotibial tract; the medial patellar and lateral patellar retinacula; and the patellar, oblique popliteal, and arcuate popliteal ligaments.

The aponeurotic tendons of the vastus medialis and vastus lateralis muscles are attached to the medial and lateral margins of the patella, down to the level of the attachment of the patellar ligament. The tendons expand over the sides of the capsule as the medial patellar and lateral patellar retinacula. They insert on the front of the tibial condyles and onto the oblique lines of the condyles as far around as the sides of the collateral ligaments. Medially, the retinaculum blends with the periosteum of the shaft of the tibia. Laterally, it blends with the iliotibial tract. Superficial to the retinacula, the fascia lata covers the front and sides of the knee joint. It descends to attach to the tibial tuberosity, and at the level of the oblique lines of the condyles, it overlies and blends with the patellar retinacula. On the lateral side, its strong iliotibial tract attaches to the oblique line of the lateral condyle and to the head of the fibula. Medially, the fascia lata is thinner and sends some longitudinal fibers inferiorly to blend with the fibrous expansion of the sartorius muscle.

The ligamentum patellae is a strong, flat band attached superiorly to the inferior pole of the patella and inferiorly to the tibial tuberosity and is actually a continuation of the tendon of the quadriceps femoris running over the anterior surface of the patella. The ligament inserts somewhat obliquely on the tibia, the lateral portion carries distally several centimeters farther than the medial portion. This longer lateral insertion may provide some protection against patellar tendon dehiscence when exposing the knee via the medial parapatellar approach. A deep infrapatellar bursa intervenes between the ligament and the bone immediately superior to the insertion. A large subcutaneous infrapatellar bursa is developed in the subcutaneous tissue over the ligament.

The oblique popliteal ligament is a posterior reinforcement of the capsule provided by the tendon of the semimembranosus muscle. As the tendon inserts into the groove on the posterior aspect of the medial condyle of the tibia (Fig. 2-5), it sends an oblique expansion laterally and superiorly across the posterior surface of the capsule toward the lateral condyle of the femur. Large foramina for vessels and nerves perforate the oblique popliteal ligament, and the popliteal artery lies against it. The arcuate popliteal ligament strengthens the lower, lateral section of the knee joint posteriorly and arises from the back of the head of the fibula, arching upward and medially over the tendon of the popliteus muscle and then spreading out over the posterior surface of the joint.