AAOS/ABOS Orthopedic Hip MCQs (Part 2): Fractures, Arthroplasty & Degenerative Hip | 2026 Board Review

Idiopathic osteonecrosis of the medial femoral condyle occurs predominantly in women older than age 60 years. It is characterized by pain centered in the medial anterior aspect of the knee, and onset is sudden. Flattening, sclerosis, and the radiolucent crescent sign are radiographic indicators of osteonecrosis. The radiographs show no narrowing of the joint space or osteophyte formation to indicate osteoarthritis, and there are no loose bodies to indicate synovial osteochondromatosis. A meniscal tear is not consistent with the radiographic findings shown here. Meniscal tears can coexist with osteonecrosis, but the pain is not eliminated merely by partial meniscectomy. Metastatic lesions to the distal femoral epiphysis are exceedingly rare. Urbaniak JR, Jones JP Jr (eds): Osteonecrosis: Etiology, Diagnosis, and Treatment. Rosemont, IL, American Academy of Orthopaedic Surgeons, 1997, pp 413-418.

PMMA has no adhesive properties and can be more accurately described as grout than glue. It does not chemically bond to bone or implants; however, mechanical bonding is accomplished with porous or coated components and with cancellous bone. PMMA is approximately three times stronger in compression than in tension. Peak blood levels of monomer are usually seen approximately 3 minutes after the cement is placed. The monomer is cleared by the lungs. Associated hypotension is more closely related to diminished blood volume than to circulating monomer levels. High porosity decreases the tensile and fatigue properties of cement. Manually mixed cement may have porosity as high as 27%. Porosity may be reduced to less than 1% through vacuum mixing or centrifugation of the cement. When adding antibiotics to cement, the compressive and tensile forces are not appreciably decreased, but the overall fatigue strength may be reduced. Canale ST (ed): Campbell's Operative Orthopaedics, ed 9. St Louis, MO, Mosby, 1998, pp 221-224.

The first rule of total knee arthroplasty is to restore the joint line to its original location. This will ensure optimal patellofemoral biomechanics and will facilitate ligament balancing. Changes on the tibial side affect both the flexion and extension gaps equally. Changes in femoral component sizing or position affect the flexion gap only. Tibial changes affect both the flexion and extension gaps. To convert a tight extension gap to a normal flexion gap, more distal femur needs to be resected. Vince KG: Revision knee arthroplasty technique. Instr Course Lect 1993;42:325-339.

A total hip arthroplasty using the posterior approach has resulted in hip dislocation under certain circumstances. Reconstruction of the external rotator/capsular complex is recognized as a stability-enhancing mechanism for the posterior approach. During the procedure, the acetabular component should be placed in 15 to 20 degrees of anteversion and approximately 45 degrees of abduction. Relative retroversion is a risk factor for posterior dislocation. High abduction angles result in edge loading of the polyethylene and possible early failure, as well as an increased risk of dislocation. Smaller diameter heads and skirted neck extensions used together decrease the range of motion that is allowed before impingement occurs, and this can result in dislocation. Shorter neck lengths generally result in soft-tissue envelope laxity. If laxity occurs, increased offset, neck length, or both can improve stability. Pellicci PM, Bostrom M, Poss R: Posterior approach to total hip replacement using enhanced posterior soft tissue repair. Clin Orthop 1998;355:224-228.

Oxidative degradation of polyethylene occurs as a function of time in an air environment. In an environment such as argon, nitrogen, or a vacuum, the process is reduced. Ethylene oxide is an alternative for sterilization in which the cross-link degradation is minimized because of the absence of oxidative interactions. Gamma sterilization or use of ethylene oxide gas is the industry standard; however, oxygen concentrations are now reduced to a minimal level to retard the oxidation phenomenon. Sanford WM, Saum KA: Accelerated oxidative aging testing of UHMWPE. Trans Orthop Res Soc 1995;20:119. Sun DC, Schmidig G. Stark C, et al: On the origins of a subsurface oxidation maximum and its relationship to the performance of UHMWPE implants. Trans Soc Biomater 1995;18:362. Callaghan JJ, Dennis DA, Paprosky WA, Rosenberg AG (eds): Orthopaedic Knowledge Update: Hip and Knee Reconstruction. Rosemont, IL, American Academy of Orthopaedic Surgeons, 1995, pp 35-41.

10b 10c In patients with infected implants, treatment usually involves debridement and exchange of the infected components. In rare cases, when there is severe comorbidity and immune system compromise, as there is with this patient, a form of chronic suppression is indicated. This patient's function is quite satisfactory and, even though there is only a 21% to 38% chance of success (Hirawaka as quoted by Mulvey and Thornhill), an attempt at suppression therapy is indicated. The patient must be followed closely to monitor the potential complications of long-term antifungal therapy and to monitor the integrity of the joint, looking for bone or soft-tissue destruction. Because the patient has satisfactory motion and quadriceps strength, no bracing or other assistive device (except for the walker he is now using) is indicated.

The theoretic total knee components will resist anteroposterior motion by making the femoral component "climb the walls" of the tibial component. As drawn, there is no constraint to medial-lateral translation. The cylinder is not rounded on the edges, so varus-valgus motion will impart load from the cylinder to the trough over a small area, thus having a high contact stress.

One of the most common complications of hemiarthroplasty is acetabular cartilage degeneration, resulting in increasing pain. Conversion total hip arthroplasty generally is successful with placement of an acetabular cup. Additionally, many patients with osteonecrosis already have degenerative changes of the acetabular cartilage even though radiographic findings may appear normal. Steinberg ME, Corces A, Fallon M: Acetabular involvement in osteonecrosis of the femoral head. J Bone Joint Surg Am 1999;81:60-65.

A 32-mm head design results in less linear wear but more volumetric wear particles. Modular components that allow motion between the polyethylene insert and the shell can result in backside wear. The oxidative degradation of gamma-irradiated polyethylene stored in air leads to increased wear. All of these factors lead to a greater particulate load and more osteolysis. Circumferential porous coating blocks ingrowth of particle-laden fluid and decreases osteolysis. Bartel DL, Bicknell VL, Wright TM: The effect of conformity, thickness, and material on stresses in ultra-high molecular weight components for total joint replacement. J Bone Joint Surg Am 1986;68:1041-1051. Fisher J, Hailey JL, Chan KL, et al: The effect of aging following irradiation on the wear of UHMWPE. Trans Orthop Res Soc 1995;20:12.

The decreased mobility of the polymer chains from cross-linking leads to decreased volumetric wear but also to decreases in ductility and fatigue resistance. Stresses at the knee are higher and varied in the point of application, leading to the concern for fatigue resistance and fracture.

In a study of 52 patients with infected total knee arthroplasties, Windsor and associates showed that the average leukocyte count was 8,300/mm3 and that aspirated knee fluid was positive in all patients except one. Knee radiographs can be unclear in showing infection, which may be present without radiographic signs of loosening. Technetium Tc 99m and gallium bone scans may not conclusively show the presence of infection, particularly in the first 3 years after knee arthroplasty. Windsor RE, Bono JV: Infected total knee replacements. J Am Acad Orthop Surg 1994;2:44-53.

12b 12c The radiographs show a halo-like sclerotic margin around the tibial stem and lucency under the baseplate. The bone scan shows markedly increased uptake under the tibial component, particularly on the medial side (not diffusely through the knee as seen with infection). These studies indicate lack of bone ingrowth fixation of the cementless porous-coated tibial component. The recent report of Fehring and associates has identified failure of ingrowth of a porous-coated implant as a dominant mode of early failure of total knee arthroplasties. Fehring TK, Odum S, Griffin WL, Mason B, Nadaud M: Early failures of total knee arthroplasty. Clin Orthop 2001;392:315-318.

A patient with an infected total hip arthroplasty may lack the symptoms of fever, chills, redness, or increased warmth typical of septic arthritis. Sensitivity for ESR and CRP ranges from 61% to 96%, and specificity ranges from 85% to 100%. Technetium Tc 99m bone scans are costly and time-consuming and will not differentiate between septic and aseptic loosening. Hip aspiration has a false-positive rate of up to 15%, although it may be useful in this patient to further complement the clinical picture if the ESR and CRP are elevated. The WBC count is rarely elevated in infected total hip arthroplasty. MRI is expensive and is not indicated for the diagnosis; however, it can aid in identifying intrapelvic extension of a periprosthetic abscess. Beaty JH (ed): Orthopaedic Knowledge Update 6. Rosemont, IL, American Academy of Orthopaedic Surgeons, 1999, pp 455-492.

The so-called third-generation cement technique adds porosity reduction techniques, centralization devices, and surface modifications to the femoral component. The surgeon must be aware of the meaning of the various generations of cement technique when interpreting the results presented at meetings and in the literature. Pellicci PM, Tria AJ Jr, Garvin KL (eds): Orthopaedic Knowledge Update: Hip and Knee Reconstruction 2. Rosemont, IL, American Academy of Orthopaedic Surgeons, 2000, pp 181-193.

Poor survivorship of cemented sockets in young patients has lead to the development of a variety of cementless designs. Of these, smooth metal-backed sockets have not performed as well as porous-coated designs. Threaded metal-backed sockets showed a 6% to 25% revision rate secondary to aseptic loosening at a mean follow-up of 4.5 to 6 years. Despite some early failed designs, cementless porous-coated metal-backed sockets have shown the best survivorship in long-term studies. Smith SE, Harris WH: Total hip arthroplasty performed with insertion of the femoral component with cement and the acetabular component without cement: Ten to thirteen-year study. J Bone Joint Surg Am 1997;79:1827-1833.

Urbaniak and Harvey reported donor site morbidity following free vascularized fibular graft in 198 consecutive patients. At a 5-year follow-up, they reported overall complications in 24% of the patients. The most common complication was a sensory deficit (11.8%), followed by motor weakness (2.7%), flexor hallucis longus contracture (2%), and deep venous thrombosis (less than 1%).

Recent literature supports retention of well-fixed cemented femoral components when revising loosened cemented acetabular components. Current literature also supports the use of cementless components for revision of loosened cemented acetabular components. Peters CL, Kull L, Jacobs JJ, Rosenberg AG, Galante JO: The fate of well fixed cemented femoral components left in place at the time of revision of the acetabular component. J Bone Joint Surg Am 1997;79:701-706. Poon ED, Lachiewicz PF: Results of isolated acetabular revisions: The fate of the unrevised femoral component. J Arthroplasty 1998;13:42-49. Moskal JT, Shen FH, Brown TE: The fate of stable femoral components retained during isolated acetabular revision: A six- to twelve-year follow-up study. J Bone Joint Surg Am 2002;84:250-255.

Contraindications to primary total knee arthroplasty include active infection, an incompetent extensor mechanism, compromised vascularity in the extremity, and local neurologic disruption affecting the competence of the musculature about the knee. Anterior cruciate, posterior cruciate, or lateral ligament incompetence can be managed with primary total knee arthroplasty. Mild flexion contracture and previous high tibial valgus osteotomy are not contraindications to primary total knee arthroplasty.

Embolization of fat and bone marrow elements during total hip arthroplasty has been studied intraoperatively using transesophageal echocardiography. These studies showed the occurrence of a large number of embolic events during the insertion of a cemented femoral stem. Embolic events were rare during insertion of a cementless stem. Femoral broaching caused some embolic events, but they were not nearly as significant as those that occurred following insertion of a cemented stem. Additionally, relocation of the cemented hip was accompanied by significant embolic events. This may be related to the untwisting of blood vessels, with the subsequent release of emboli that were most likely generated during insertion of a cemented femoral stem. Pitto RP, Koessler M, Kuehle JW: Comparison of fixation of the femoral component without cement and fixation with use of a bone-vacuum cementing technique for the prevention of fat embolism during total hip arthroplasty. J Bone Joint Surg Am 1999;81:831-843.

Prophylactic LMWH is associated with a risk of bleeding complications, especially if administered too soon after surgery. The risk of major bleeding is 0.3% for control, 0.4% for aspirin, 1.3% for warfarin, 1.8% for LMWH, and 2.6% for unfractionated heparin. Colwell and associates conducted a prospective, randomized trial on over 1,500 total hip arthroplasty patients. Overall, the risk of clinically apparent venous thrombolembolism was 3.6% for LMWH and 3.7% for warfarin. LMWH acts in several sites of the coagulation cascade, with its principal action being inhibition of factor 10a. Thrombocytopenia is less common with LMWH than with unfractionated heparin. The use of LMWH is a relative contraindication with indwelling epidural anesthesia. Colwell CW Jr, Collis DK, Paulson R, et al: Comparison of enoxaparin and warfarin for the prevention of venous thromboembolic disease after total hip arthroplasty:. Evaluation during hospitalization and three months after discharge. J Bone Joint Surg Am 1999;81:932-940.

Multiple studies have shown that the size of an ultra-high molecular weight polyethylene particle generated by total hip implants is typically less than 1 micron. This finding is significant in that particles of that size are readily phagocytized by macrophages. Campbell P, Ma S, Yeom B, McKellop H, Schmalzried TP, Amstutz HC: Isolation of predominantly submicron-sized UHMWPE wear particles from periprosthetic tissues. J Biomed Mater Res 1995;29:127-131. Shanbhag AS, Jacobs JJ, Glant TT, Gilbert JL, Black J, Galante JO: Composition and morphology of wear debris in failed uncemented total hip replacement. J Bone Joint Surg Br 1994;76:60-67.

Risk factors can suggest the existence of osteoporosis. However, definitive testing, based on the use of bone densitometry measurements, uses the T score in which an average score is taken from a normal population of young women. The presence of increased osteoid in lamellar bone is seen in osteomalacia but not osteoporosis. The presence of fractures is evidence of a risk factor for osteoporosis and can predict future fractures, but it does not definitively confirm the diagnosis. The Singh index is a radiographic finding that is not as accurate as bone mineral density scores.

Joshi and associates reported a 96% incidence of pain relief in 181 total hip arthroplasties in patients with ankylosing spondylitis. Only 65% of patients had good to excellent functional results, primarily the result of associated systemic diseases and spinal deformity. The incidence of infection was slightly higher, and the incidence of heterotopic ossification was higher in this group of patients.

Parvizi and associates studied the 30-day mortality rate after more than 22,000 total knee arthroplasties and found that the rate after bilateral total knee arthroplasty was significantly higher than after unilateral total knee arthroplasty. Aseptic loosening, bleeding, and range of motion have not been shown to be statistically different between patients who had unilateral and simultaneous bilateral total knee arthroplasty.

The improvements in metal-on-metal bearing surfaces come from the nonlinear wear rate and smaller particle size of the high carbon wrought material. Extremely low rates of wear have been demonstrated with high carbon metal-on-metal implants. There is no significant electrochemical effect of mating two like materials in vivo.