Adult Hip and Knee Reconstruction

6


Adult Hip and Knee Reconstruction


Scott Ritterman, John Froehlich, and Matthew Miller


I. Total Hip Replacement


1. Anatomy


• Ball-and-socket joint


a. Socket deepened by thick flexible labrum


b. Cotyloid fossa: floor of acetabulum, no cartilage, ligamentum teres


Radiographic marker “teardrop” is formed by:


♦ Quadrilateral plate from inside pelvis


♦ Cotyloid fossa from floor of acetabulum


c. Adult’s main blood supply to femoral head from medial femoral circumflex artery


• Surgical approaches


a. Anterior (Smith-Petersen approach)


Internervous plane


♦ Sartorius (femoral nerve) and tensor fascia lata (superior gluteal nerve) superficially


♦ Gluteus medius (superior gluteal nerve) and rectus femoris (femoral nerve) deep


Ascending braches of the lateral circumflex artery must be cauterized or ligated.


Lateral femoral cutaneous nerve typically exits the pelvis laterally just inferior to the inguinal ligament, but there are several anatomic variants.


b. Anterolateral (Watson Jones)


Intermuscular plane


♦ Tensor fascia lata and gluteus medius (superior gluteal nerve)


♦ Traditionally requires trochanteric osteotomy for adequate exposure of the hip joint


c. Direct lateral


No intermuscular plane


♦ Splits anterior third of gluteus medius and minimus, and anterior portion of vastus lateralis


Dislocate hip anteriorly


Can lead to Trendelenburg gait postoperatively due weakness of gluteus medius and possible damage to superior gluteal nerve


♦ Superior gluteal nerve lies 3–5 cm proximal to greater trochanter on the underside of the gluteus medius muscle


♦ Prone to injury by stretching or transection


d. Posterior approach


No intramuscular plane


♦ Split gluteus maximus muscle (inferior gluteal nerve)


♦ Tag and divide piriformis tendon; alternatively, can spare tendon and retract


♦ Short external rotators are detached and reflected with posterior capsular flap used to protect the sciatic nerve during dissection


♦ Hip extension and knee flexion will protect the sciatic nerve from excess stretch intraoperatively.


Increased dislocation risk if capsule is not repaired with the posterior approach



Most common reason for sciatic nerve injury is errant retractor placement. Hematoma can form postoperatively as a result of postop anticoagulation, which can cause sciatic palsy as well. Treatment is evacuation of hematoma. Females, revision surgery, and patients with severe hip dysplasia (congenital dislocation) have highest risks of nerve injury.


2. Physical examination


• Normal range of motion (ROM)


a. Flexion: 135 degrees


b. Extension: 10–15 degrees


c. Abduction: 45 degrees


d. Adduction: 30 degrees


e. Internal rotation: 35 degrees


f. External rotation: 45 degrees



• Hip rotation can be checked in both flexion and extension. Excessive femoral anteversion results in increased internal rotation in extension, which may not be obvious or present in flexion.


• Motor function


a. Hip flexion against resistance from a seated position


Test for iliopsoas impingement (snapping hip) with the patient seated, pain with resisted flexion


b. Adduction


c. Abductors: lay patient on contralateral side and keep leg abducted against resistance


d. Knee extension against resistance (quad strength)


• Gait examination


a. Antalgic: shortened stance phase, unloading painful limb


b. Abductor lurch (Trendelenburg gait)


When the abductors are deficient, the pelvis tilts down toward the contralateral side during single-limb stance. In compensation, the trunk leans toward the deficient side to maintain the center of gravity over the weight-bearing limb. See Trendelenburg test, below.


• Leg length discrepancy (LLD)


a. Always should be evaluated preoperatively and intraoperatively


Common cause of litigation


b. Some people have small LLD at baseline and most can tolerate up to 1 cm difference


c. True LLD


Actual lengthening measured with anteroposterior (AP) pelvis, comparing distance between lesser trochanter and transischiac line, teardrop line, or some other fixed point (Fig. 6.1)


d. Apparent LLD: perceived leg length difference


Hip flexion/adduction contracture


Scoliosis, pelvis obliquity


Lengthening after long-term shortened limb (chronic femoral neck fracture, acetabular protrusio)


e. Measurements


Anterior superior iliac spine (ASIS) to medial malleolus


Umbilicus to medial malleolus


• Special tests


a. Trendelenburg test (Fig. 6.2)


To test the left abductors, ask the patient to stand on the left leg. If the patient’s pelvis tilts to the right, then the test is positive, indicating weak/deficient abductors of the left hip.



b. Thomas test (Fig. 6.3)


To test for a hip flexion contracture, ask the patient to lie in the supine position, and, placing one hand under the patient’s lower back to prevent excessive lumbar lordosis, flex the unaffected hip as much as possible. Lift the patient’s affected limb and release. The leg should return to the examination table with the lower back in contact with the examiner’s hand.


c. Impingement test


Flex hip to 90 degrees


Pain with internal rotation and adduction


d. Labral shear test


Pain with circumduction of the hip (like McMurray maneuver for the hip)


• Differential diagnosis of hip joint pain


a. Trochanteric bursitis/tight iliotibial band (ITB)


Tenderness centered over greater trochanter; check Ober test



b. Sacroiliac (SI) joint inflammation


c. FABER (flexion, abduction, and external rotation) test: pain with flexion, abduction, external rotation in the SI joint region


d. Lumbar radiculopathy (see Chapter 5)


Radicular symptoms extending beyond the knee


Evaluate with lumbosacral spine X-ray and possibly magnetic resonance imaging (MRI)


e. Intrapelvic pain


Hernia


Pelvic inflammatory disease (PID)


Diverticular disease



f. Diagnostic hip injection with local anesthetic can help differentiate between intracapsular and extracapsular causes of pain.



3. Hip disease


• Dysplasia


a. Developmental malalignment of the hip joint


b. Typically presents in fourth to seventh decades with arthritis of the hip joint


c. May be subtle or pronounced


Abnormal biomechanics lead to degenerative changes within the hip joint.


Time of presentation depends on the degree of dysplasia.


d. Acetabular dysplasia


Coverage


♦ Under-coverage of femoral head: typically anterior and laterally


▪ Lateral center-edge angle (Fig. 6.4)


▪ Normally 25–40 degrees; < 20 degrees is typical in developmental dysplasia of the hip (DDH)


♦ Over-coverage of femoral head


▪ Center-edge angle > 40 degrees can lead to pincer type impingement as the hip flexes and abducts.


Version


♦ Acetabular retroversion (normally anteverted)


♦ Crossover sign


▪ Radiographic cardinal lines/landmarks (Fig. 6.5)


▪ When anterior wall crosses posterior wall medial to the weight-bearing lateral rim


▪ May be present in those with pincer-type femoroacetabular impingement (FAI)


e. Femoral dysplasia


Head–neck dysplasia (also see Chapter 8)


♦ Cam lesion reduces the native head/neck ratio, leading to early femoral neck impingement during normal ROM


♦ Pistol grip deformity seen on radiograph


♦ Alpha angle: formed by a line drawn from the center of the femoral head through the center of the femoral neck and a line drawn from the center of the femoral head to the head/neck junction; alpha angle > 50–55 degrees indicates a likely cam lesion


♦ Demonstration of alpha angle (Fig. 6.6)




f. FAI (also see Chapter 8)



Abnormal impingement can be from the acetabulum or from the femur; evaluate the radiograph to delineate the etiology of the impingement.


Abnormal contact between anterior rim of acetabulum and femoral neck, leading to block in motion and pain, decreased internal rotation


Pincer type: labrum caught between bony surfaces


♦ Occurs with acetabular retroversion or over-coverage


▪ Crossover sign seen on X-ray


♦ Labrum is damaged


Cam type: raised area of proximal femur impinges on anterior acetabulum during hip flexion


Combined: features of both cam and pincer type


♦ Damage to chondral surface of acetabulum


Presentation


♦ Pain with hip flexion and internal rotation


♦ Decreased internal rotation or obligate external rotation with flexion



Treatment: aimed at cause of dysplasia


♦ Ganz periacetabular osteotomy (PAO); used to correct acetabular dysplasia


▪ Osteotomize around the acetabulum while leaving the posterior column intact to redirect the native acetabulum.


▪ Goal is to medialize the joint and gain lateral/anterior coverage of the femoral head.


• Joint medialization decreases joint reactive forces.


• Correction of acetabular version, lateral center-edge angle


♦ Anterior hip decompression


▪ Arthroscopic versus open



Nerve injury is the most common complication of hip arthroscopy. The pudendal nerve is at risk due to traction needed to access the hip. Traction should not be applied for longer than 2 hours. The pudendal nerve exits the pelvis through the greater sciatic notch, and reenters through the lesser sciatic notch. It provides sensation to external genitalia


▪ Debridement of cam lesion


▪ Debridement of anterior acetabulum (rim trim)


▪ Repair/debride torn labrum


♦ Proximal femoral osteotomy


▪ Varus producing osteotomy


• Typically done after PAO with insufficient lateral coverage (insufficient center-edge angle)


▪ Valgus producing osteotomy


• Typically for femoral neck nonunion in adults


♦ Total hip considerations with hip dysplasia


▪ Acetabulum is shallow, deficient superiorly and anteriorly.


▪ Femoral neck may be short with excessive anteversion.


▪ Proximal migration of femoral head may exist (chronic shortening of leg and neurovascular structures).


• Insufficient abductors may need to be transferred distally with trochanteric osteotomy.


• Shortened hamstrings and adductors may need to be lengthened.


• Shortened sciatic nerve



• Sciatic nerve palsy may occur after excessive lengthening during total hip arthroplasty (THA).


images Susceptible to injury if leg lengthened > 2–3 cm, although can occur with less


images May require femoral shortening osteotomy (subtrochanteric)


• Osteonecrosis



• The medial femoral circumflex artery is the dominant blood supply to the femoral head in adults.


a. Loss of blood supply to femoral head


b. Posttraumatic


Femoral head necrosis after femoral head or neck fracture, or traumatic or iatrogenic hip dislocation


c. Idiopathic: avascular necrosis (AVN)


Demographics: third and fourth decades of life


Presentation: acute or progressive groin pain, decreased/painful internal rotation and flexion



Risk factors:


♦ Alcoholism


♦ Corticosteroids


♦ Hypercoagulability


▪ Sickle cell anemia


• 75% of asymptomatic patients with osteonecrosis develop symptoms/collapse if untreated


Imaging


♦ Radiographs: AP pelvis, AP lateral hip


▪ Look for sclerosis or cystic changes in femoral head


▪ Crescent sign (Fig. 6.7)


▪ Collapse of femoral head


▪ Ficat classification (Table 6.1)


♦ MRI


▪ Edema within the femoral head/neck


▪ Always check contralateral hip for asymptomatic disease.


Treatment


♦ Nonsurgical


▪ Activity modification: limited weight bearing


▪ Weight loss


Table 6.1 Ficat Classification






















Grade


Findings


0


Pain without radiographic or MRI findings


I


Normal radiograph with edema within femoral head on MRI


II


Radiograph with sclerotic changes to femoral head


III


Femoral head flattening with crescent sign on radiograph


IV


Femoral head collapse, loss of joint space, osteoarthritis


▪ Nonsteroidal anti-inflammatory drugs (NSAIDs)


▪ Bisphosphonates: can prevent/delay collapse if given in the early stages


♦ Surgical


▪ Core decompression


• Tunnel(s) drilled up the femoral neck into the head to allow decompression and revascularization; often combined with bone grafting


• May be effective early in disease before crescent sign and collapse occur (Ficat grades I and II)


▪ Vascularized fibular graft


• Free tissue transfer to place vascularized autograft into the femoral head


• Pain at donor site/leg following vascularized fibula transfer may indicate a tibial stress fracture; evaluate with MRI.


▪ Total hip replacement


• Most reliable treatment for AVN with collapse



Can also consider hip fusion in young laborer


• Young patient with Ficat grade III or IV (collapse present)


• Osteoarthritis


a. Progressive cartilage loss leads to weight bearing on subchondral bone


Demographics: fourth to eighth decades of life; females > males


Presentation: progressive groin pain with weight-bearing activities; decreased rotation; external rotation and flexion contractures due to long-standing joint inflammation


Imaging


♦ Weight-bearing AP pelvis, AP hip, cross-table lateral of hip


Treatment


♦ Nonsurgical


▪ Activity modification with continued low-impact exercise


▪ Weight loss


▪ NSAIDs


▪ Crutch/cane: placed in contralateral hand will off-load weight during stance phase


▪ Intra-articular corticosteroid injection (fluoro or ultrasound guided)


♦ Surgical


▪ Hip fusion



• Ideal position is 0–5 degrees external rotation, 0–5 degrees adduction, 20–35 degrees flexion


• Typical candidate is a young male laborer with debilitating arthritis who needs to continue working


• Unilateral disease


• One-third increase in energy needed to ambulate


• As with any fused joints, increased stresses are translated to adjacent joints, which become arthritic (spine, knee)


▪ Fusion to THA



• Must have hip abductors present to convert a hip fusion to an arthroplasty for stability; otherwise, will require a constrained liner for stability


• Often done for adjacent joint disease (lumbosacral spine most commonly, knee)


• Function directly related to integrity of abductor complex


• Preoperative electromyogram (EMG) required to test abductors


• Deficient abductors require constrained hip liner or tripolar articulation


▪ Hip resurfacing arthroplasty


• Intended for active young male patients


• Larger head should confer greater static stability


• Metal-on-metal resurfacing of femoral head and acetabulum


• Advantages include preservation of bone stock, lower wear without polyethylene liner, retain large femoral head size and therefore lower dislocation risk


• Contraindications


images Osteoporosis, coxa vara, femoral neck cyst


images Abnormal acetabular anatomy


images Significant leg length discrepancy


images Smaller anatomy


images Renal disease


images Metal allergy


• Complications


images Periprosthetic femoral neck fracture up to 4% and is the most common reason for revision before 20 weeks


images Metal-on-metal debris is generated, leading to T-cell immune response; see Metal-on-metal wear, below.


• Hemiarthroplasty


a. Typically used for femoral neck fractures in low demand, elderly patients



b. Unipolar hemiarthroplasty refers to a one-piece head that is attached to the neck of the implanted femoral component. A bipolar hemiarthroplasty refers to an articulating head within the larger head component. In theory, this more equally distributes shear forces, leading to less wear of the native acetabulum as well as a greater range of motion. However, motion within the inner bearing decreases over time. There is no difference in outcomes.


c. Low demand, infirm patient without antecedent hip/groin pain


d. Advantage is large head size, allowing for more stability and lower dislocation rate


Preserve the labrum, repair hip capsule for greatest postoperative stability


e. Active patients develop groin pain due to chondrolysis and have superior outcome with total hip replacement


• Total hip arthroplasty


a. Use THA for active patients with arthritis and displaced femoral neck fracture.


b. Higher dislocation rate with femoral neck fracture than elective THA for osteoarthritis (OA)


4. Basics of total hip replacement


• Bearing surfaces


a. Soft: ceramic on polyethylene (PE), metal on PE


b. PE: hydrocarbon molecule


Ultra-high molecular weight polyethylene (UHMWPE) has been in use for > 40 years


♦ Mechanical properties depend on percentage of PE in


▪ Amorphous phase


▪ Crystalline phase


♦ Highly cross-linked UHMWPE


▪ Better wear characteristics


♦ Significantly reduces wear compared with conventional UHMWPE


▪ Less mechanical strength


♦ Need to find compromise between the two



Steps in PE liner production: 1, manufacture; 2, sterilize; 3, cross-link; 4, melt/anneal; 5, package


Manufacturing


♦ Direct compression molding from powder has the best wear characteristics.



During manufacturing via ram-bar extrusion, calcium stearate was once added to PE to prevent sticking to equipment; results in increased wear, and reduced mechanical properties; no longer used but still tested


Sterilization


♦ Low-dose irradiation (2.5–4 Mrd) is best mode


♦ Irradiation (at higher dose) also allows cross-linking to occur


Cross-linking of hydrocarbon chains between molecules of PE provides greater wear resistance.



Why cross-link? More wear resistant, more resistant to adhesive and abrasive wear; smaller wear particles


Irradiation of PE creates free radicals that can bind either:


♦ Oxygen: oxidized PE, chain scission, and no cross-linking occurs; greatly increased wear


♦ Other PE molecules: in an oxygen-free environment (typically in the presence of inert gases such as argon), free radicals bind other PE molecules and form cross-links (which is good)


Therefore, irradiation must be done in an oxygen-free environment!



Sterilization of polyethylene and packaging in air lead to premature polyethylene wear and osteolysis, and is a commonly tested concept.


Irradiation in larger amounts of radiation (5–20 Mrad) creates highly cross-linked polyethylene


♦ Better wear resistance than cross-linking with lower dose of irradiation


♦ Decreased mechanical strength compared with regular cross-linked PE, more brittle


♦ Smaller wear particles


Heating of PE after cross-linking is required to remove excess free radicals. Melting removes all free radicals; annealing removes some.


♦ Affects the structure of PE


▪ Crystalline: ideally 45–65% of PE


• Greater percentage of crystallinity leads to greater mechanical strength


▪ Amorphous


• Where cross-linking occurs


♦ Melting


▪ Less free radicals, better resistance to oxidation in vivo


▪ Reduces mechanical properties; lowers crystallinity


♦ Annealing: heating PE to less than melting point


▪ Better mechanical properties than melting (decreased wear) due to higher crystallinity


▪ Higher risk of free radicals and resulting oxidation in vivo


Vitamin E can be added to reduce free radicals by blending or infusion.


Shelf life: vacuum-sealed package


♦ Remaining free radicals have the potential to oxidize while PE sits on the shelf awaiting use.


♦ Concern about odd-sized components that may sit on shelf for extended time



Two reasons for irradiating: (1) sterilization (2.5–4 Mrad); (2) creation of free radicals for cross-linking to form highly cross-linked PE (5–20 Mrad)


All PE components regardless of means of sterilization will begin to oxidize once implanted and exposed to environment.


PE wear products lead to osteolysis (see below)


c. Hard bearing surfaces


Metal


♦ Alloys consisting of cobalt, chrome, molybdenum, nickel, and other substances are used for bearing surfaces due to their resistance to corrosion.


♦ Titanium is too soft to use as a bearing surface but has a stiffness similar to bone and is therefore ideal for use in both femoral implant and acetabular shell.


▪ Titanium has a Young modulus of 115 GPa.



• Relative Young’s modulus of common materials from highest (stiffest) to lowest:


images Ceramic


images Cobalt-chromium (CoCr)


images Steel


images Titanium


images Cortical bone


images Tantalum


images Cement


images Polyethylene


images Cancellous bone


images Cartilage


♦ Scratches too easily, leading to increased wear if used for bearing surface (head)


Theoretically, metal-on-metal (MOM) bearings have a lower wear rate, generate smaller wear particles compared with PE bearings, and allow the use of a large head, conferring greater joint stability.


Imprecise placement of components, especially an over-abducted or anteverted cup, can lead to edge loading and generation of a large number of wear particles, increasing serum cobalt and chromium ion concentrations, and stimulating a T-cell response (see Metal-on-metal wear, below).


Ceramic: alumina ceramic and zirconia ceramic


♦ Ceramic on ceramic (COC)


▪ Low wear


▪ Fewer particles than in MOM


▪ Bio-inert debris


▪ Limited head size, less optimal fluid film mechanics (see below)


▪ Can entail squeaking, possibly due to component malposition


▪ Risk of fracture


• First-generation ceramic implants susceptible to fracture, due to manufacturing imperfections and brittle materials


• Current-generation ceramic implants have significantly lower fracture rates


• Low toughness (limited plastic deformation) (see Chapter 12)


• After fracture, must revise with COC bearing



• When a ceramic component fractures, revision must always be with COC. Small fragments always remain and would cause rapid PE wear.


• When exchanging prosthetic heads for any reason and retaining femoral components, a jacket should be placed on the retained trunnion. A new ceramic head cannot be placed directly on an old trunnion, as it will lead to fracture of the new femoral head.


Advantages of hard-on-hard bearings


♦ Potential to decrease osteolytic wear, which was a main issue with conventional PE


♦ Much smaller wear particles generated for MOM or ceramic components


▪ 0.015–0.12 µm compared with 0.2–7 µm for hard on soft (polyethylene)


▪ Smaller particles are not recognized by macrophages.


▪ Immune response is mediated by lymphocytes.


MOM hips have very low wear rate but generate many more particles (see below)


d. Lubrication


Boundary


♦ Occurs while at rest and slow walking


♦ Two bearing surfaces in contact


Hydrodynamic


♦ Two bearing surfaces are completely separated by fluid.



Lambda ratio


Takes into account roughness, head size, viscosity, angular velocity


Greater than 3 indicates fluid film mechanics


♦ More smooth bearing surfaces leads to hydrodynamic lubrication


♦ Larger head size


▪ ≥ 38 mm is most likely to achieve hydrodynamic lubrication


♦ Requires angular motion to achieve hydrodynamic lubrication; must be walking


▪ Surface roughness


♦ Bearing surfaces must be very smooth


▪ Ceramic > metal > PE


• Ceramic Ra < 0.01 µm


• Metal Ra 0.01 µm


• PE Ra several µm


e. Sphericity


Variation leads to small high points and localized stress points, which negatively affects lubrication.


Measured as “out of roundness” in µm


9- to 10-µm heads have more PE wear than 0.5-µm heads.


f. Radial clearance


Difference in radius of curvature of the head and the cup


♦ Equatorial contact


▪ Head is larger than the cup.


▪ High friction


• No space for fluid to enter or exit


♦ Polar contact


▪ Head is smaller than the cup.


▪ One point of contact


▪ High stress at point of contact and poor lubrication


♦ Midpolar contact (ideal)


▪ Radius of curvature of head is slightly smaller than that of cup.


▪ Fluid is able to enter the joint and lubricate the bearing surfaces.


▪ Cannot have complete congruence or fluid cannot enter to lubricate joint


g. Wear


Any process that leads to breakdown of bearing surfaces, resulting in particulate debris, increased friction, and altered biomechanics


Volumetric wear


♦ Calculated, though exact equation is debated


♦ Directly related to size of prosthetic head


♦ Larger heads lead to greater volumetric wear


Linear wear


♦ Wear rates above 0.1 mm per year are at high risk of osteolysis.


▪ Any new polyethylene with wear rates < 0.1 mm/year should have minimal osteolysis (> 10 billion particles/gram of tissue)


♦ Measured on radiograph


♦ Penetration of femoral head into the liner



Smaller heads have greater linear wear, less volumetric wear


Adhesive wear


♦ PE particles pulled off form liner during gait cycle


♦ Most important in generation of PE debris in hips



Cross-linking of PE has led to decreased adhesive and abrasive wear and has significantly decreased the risk of osteolysis.


Abrasive wear


♦ Rough femoral head scratches and mechanically damages PE liner


Third body wear


♦ Any material between the two bearing surfaces; cement, metal shavings, bone, etc.


♦ Debris between two bearing surfaces wears/scratches the weaker one


Run-in wear


♦ Higher wear rate within the first 1 million cycles of THA use (bedding in period)


♦ Decreases thereafter as it goes into steady-state phase


♦ High stress points on surfaces polished out


Stripe wear


♦ Seen with ceramic prosthetic heads


♦ Occurs with lift-off separation when the femoral head contacts the rim of the shell as it separates from the socket


♦ Crescent-shaped line on femoral head and corresponding on cup, near edge


♦ Surface wear 1–60 µm deep


♦ Seen more commonly in those cups that are vertically oriented


Hip edge loading: radiograph shows an over-abducted cup


♦ Stresses concentrate on edge of acetabular shell


▪ To prevent, need to make sure that midpolar contact occurs, and the cup should not be abducted past 45 degrees (Fig. 6.8)



• Implant fixation


a. Biological


Biological fixation: dynamic relationship between bone and prosthesis with ability to remodel over time


Prosthesis must be coated to allow bone ingrowth or ongrowth.


Beware of devascularized bone.


♦ Patients who have been irradiated may not be able to support bony ingrowth (requiring tantalum cup and multiple screw fixation or cement for fixation).


Press fit versus line-to-line technique


♦ Press fit refers to preparing bone (either femur or acetabulum) to a certain size and then inserting an implant that is slightly larger (1–2 mm). Compressive hoop stresses provide the initial fixation while bone ongrowth/ingrowth occurs.


♦ Line-to-line technique refers to preparing the femur or cup to a certain size and inserting the same-size implant. Acetabular shell requires screws if placed in this fashion; femoral stem is fully porous coated for initial rigid fixation.


▪ For both, the long-term fixation is biological.


Porous coating: allows for bone ingrowth


♦ Porosity


▪ Too little porosity will not leave enough room for ingrowth.


▪ Too much porosity will lead to failure due to shear forces.


▪ 40–50% porosity is ideal


• Pore depth


images Deeper is better to some extent.


• Pore size


images 50–400 µm is ideal pore size


• Micromotion


images < 30 µm for bone ingrowth


images > 150 µm leads to fibrous fixation


• Proximal or metaphyseal loading


images Stems that are coated proximally will lead to proximal ingrowth and loading of proximal bone (preventing stress shielding).


Grit blasting: allows for bone ongrowth


♦ Surface roughness


▪ Difference in height between peaks a valleys


▪ Rougher the surface, greater the fixation


♦ Typically diaphyseal fitting with more stress shielding proximally because of diaphyseal loading/stress


▪ Fully coated stems will gain fixation distally into the diaphysis.


▪ Stress shielding may occur, which leads to decreased bone density and remodeling proximally.



♦ More function of stem stiffness


▪ Thicker stems


▪ Cobalt is stiffer than Titanium, and Titanium is stiffer than cortical bone


♦ Hydroxylapatite-coated stems have shorter time frame to achieve biological fixation.


▪ Ca10(PO4)6(OH)2



Rule of 50’s for bony ingrowth:


Less the 50 µm between bone and prosthesis


Less than 50 µm of motion for good ingrowth


Pore size of prosthesis between 50 and 150 µm


Porosity no greater than 50%


b. Cemented


Static fixation, diaphyseal loading


Relies on interdigitation between cement and trabecular bone


Long-term failure in young active patients


♦ Will fatigue at stress points in mantle; does not remodel


♦ Cemented cups fail earlier than femur


▪ Too much shear and tensile forces


▪ Cement stronger in compression, fails in tension/shear


The femur must be appropriately broached and cleared of any marrow or fat debris.


A canal restrictor is placed within the femoral canal 1.5–2 cm distal to the eventual tip of the prosthesis.


The canal is back-filled with cement from distal to proximal.


♦ The cement is pressurized.


♦ Component is centralized in the cement mantle and held in position until cement hardening.


♦ Cement mantle should be at least 2 mm on every side and cement should occupy two thirds of the canal and the prosthesis the remaining one third.


♦ A mantle defect is a point where the prosthesis touches the bone and is a high stress point, susceptible to fracture.



Precoating of the femoral component with cement adds an additional interface where fixation failure can occur.


♦ Osteopenic bone has improved fixation with cemented components. Porous bone allows increased interdigitation of cement.


• Acetabular component screw placement


a. Required for line-to-line acetabular component placement


b. Line between ASIS and center of fovea splits acetabulum into halves


c. Line two is perpendicular at the center of the acetabulum, dividing into four quadrants


d. Structures at risk (Fig. 6.9):



Posterior superior is safe zone for screws

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Jun 28, 2018 | Posted by in ORTHOPEDIC | Comments Off on Adult Hip and Knee Reconstruction
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