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3. Adult Reconstruction (Hip & Knee) MCQs

Practice Set 63 of 326

This practice set contains high-yield board review questions covering key concepts in 3. Adult Reconstruction (Hip & Knee). Each clinical scenario is designed to test your diagnostic and management skills relevant to this subspecialty.

Question 1241

Topic: 3. Adult Reconstruction (Hip & Knee)
During impaction of a cementless acetabular component, the posterior column was fractured and found to be displaced. Which of the following is considered the most appropriate surgical option?
. Exchange of the cementless cup to a larger component
. Retention of the component and bone grafting of the fracture
. Retention of the component and postoperative weight protection until the posterior column heals
. Removal of the cup, fixation of the posterior column, and application of an antiprotrusio cage
. Removal of the cup and cementing of an all-polyethylene liner

Correct Answer & Explanation

. Removal of the cup, fixation of the posterior column, and application of an antiprotrusio cage

Explanation

Acetabular bone loss presents a challenge during reconstruction. A cementless hemispherical cup can be used in most patients provided that the acetabular rim, particularly the posterior column, is intact. When the posterior column is disrupted, fixation with a reconstruction plate and/or the use of an antiprotrusio cage is recommended. The latter is particularly important when the posterior column is fractured and displaced, such as in this patient. Under these circumstances, reduction of the fracture and application of an antiprotrusio cage is recommended.

Question 1242

Topic: 3. Adult Reconstruction (Hip & Knee)
Figures below show the radiographs obtained from a 68-year-old man with progressively worsening right side hip pain over the last 8 months. He is 6 feet tall, with a BMI of 51 kg/m2 and reports that his index total hip arthroplasty was performed 8 years ago. The preoperative work-up includes negative infectious laboratory results. What is the most appropriate surgical plan for revision of the femoral component in this patient?
. Superior approach with trochanteric slide
. Direct anterior approach with a chevron modification of the standard greater trochanteric osteotomy
. Lateral approach with a partial greater trochanter osteotomy
. Posterolateral approach with an extended trochanteric osteotomy

Correct Answer & Explanation

. Posterolateral approach with an extended trochanteric osteotomy

Explanation

The patient’s radiographs demonstrate varus femoral remodeling around a broken cylindrical, distally fixed femoral stem. Proximal femoral remodeling around loose or fractured stems occurs in 21% to 42% of femoral revisions. An extended trochanteric osteotomy would aid in the removal of the well-fixed distal segment and enable the safe insertion of the new femoral component. The approach is not the concern in this case, because extended trochanteric osteotomies have been described from the posterior and direct lateral approaches with excellent outcomes and union rates. The key is that the extended osteotomy is necessary and not a trochanteric slide or standard (shorter or incomplete trochanteric) osteotomy. These types would not provide access to the well-fixed distal stem, nor would they afford a straight tube in which to insert a new femoral component.

Question 1243

Topic: 3. Adult Reconstruction (Hip & Knee)
Figures 35a and 35b show the radiographs of a patient who underwent debridement of a chronically infected, fully constrained knee prosthesis and now reports pain and instability despite bracing. History reveals that the patient has had no drainage since undergoing the last debridement 6 months ago. A C-reactive protein level and aspiration are negative for infection. Treatment should now consist of:
. knee arthrodesis.
. insertion of a semiconstrained prosthesis.
. insertion of an antibiotic-impregnated polymethylmethacrylate (PMMA) spacer.
. reconstruction of the extensor mechanism.
. amputation.

Correct Answer & Explanation

. knee arthrodesis.

Explanation

The radiographs show a significant loss of the proximal anterior tibial cortex, consistent with an extensively damaged or deficient extensor mechanism. Such a deficit precludes insertion of another knee arthroplasty. Arthrodesis is the treatment of choice for this patient and is indicated for loss of the extensor mechanism and knee instability. A recent report on arthrodesis following removal of an infected prosthesis showed a union rate of 91% using a short intramedullary nail. Insertion of an antibiotic-impregnated PMMA spacer is not indicated because the rationale for using a spacer is to maintain a space for reinsertion of another prosthesis. Reconstruction of the extensor mechanism would not address the loss of the joint. Amputation is the final treatment option if the arthrodesis fails.

Question 1244

Topic: 3. Adult Reconstruction (Hip & Knee)
Which nerve is most commonly damaged with anterior total hip replacement surgery?
. Lateral femoral cutaneous nerve
. Inferior gluteal nerve
. Superior gluteal nerve
. Sciatic nerve

Correct Answer & Explanation

. Lateral femoral cutaneous nerve

Explanation

DISCUSSION: The nerve most commonly injured in the posterior approach to the hip is the sciatic nerve. Overall injury prevalence is 1% to 2%. This nerve is more commonly injured in cases of hip dysplasia with excessive leg lengthening. The superior gluteal nerve is at highest risk with the direct lateral approach to the hip. This nerve courses in the gluteus medius muscle and is at risk when splitting the muscle 5 cm proximal to the greater trochanter. The lateral femoral cutaneous nerve is commonly damaged with anterior total hip replacement surgery. Neuropraxia has been reported in 81% of patients. The inferior gluteal nerve travels from the greater sciatic notch and enters the gluteus maximus muscle. It is at risk when the posterior approach to the hip is used. RECOMMENDED READINGS: Hoppenfeld S, deBoer P. Surgical Exposures in Orthopedics. 3rd ed. Philadelphia, PA: Lippincott Williams & Wilkins; 2003:365-453. DeHart MM, Riley LH Jr. Nerve injuries in total hip arthroplasty. J Am Acad Orthop Surg. 1999 Mar-Apr;7(2):101-11. Review. PubMed PMID: 10217818. Goulding K, Beaulé PE, Kim PR, Fazekas A. Incidence of lateral femoral cutaneous nerve neuropraxia after anterior approach hip arthroplasty. Clin Orthop Relat Res. 2010 Sep;468(9):2397-404. doi: 10.1007/s11999-010-1406-5. PubMed PMID: 20524096.

Question 1245

Topic: 3. Adult Reconstruction (Hip & Knee)
Which of the following treatment regimens for thromboembolic prophylaxis meets the American College of Chest Physicians Guidelines for 10-day treatment after total hip arthroplasty and total knee arthroplasty?
. Low-molecular-weight heparin
. Adjusted dose unfractionated heparin
. Aspirin
. Warfarin, INR 1.5 to 2.0
. Elastic compressive stockings

Correct Answer & Explanation

. Low-molecular-weight heparin

Explanation

DISCUSSION: Only three thromboembolic treatment protocols have reached Grade 1A status for the American College of Chest Physicians Guidelines for thromboembolic prophylaxis after total hip arthroplasty and total knee arthroplasty. Grade 1A evidence shows a clear benefit/risk improvement with supportive data from randomized clinical trials, which are strongly applicable in most clinical circumstances. Warfarin is recommended but at an INR level of 2 to 3. Low-molecular-weight heparin and fondaparinox are also acceptable treatment options. Aspirin, adjusted dose unfractionated heparin, and elastic compressive stockings are not recommended as stand-alone options. REFERENCES: Colwell C: Evidence based guidelines for prevention of venous thromboembolism: Symposia. Proceedings of the 2005 AAOS Annual Meeting. Rosemont, IL, American Academy of Orthopaedic Surgeons, 2005, pp 15-18. Freedman KB, Brookenthal KR, Fitzgerald RH, et al: A meta-analysis of thromboembolic prophylaxis following elective total hip arthroplasty. J Bone Joint Surg Am 2000;82:929-938.

Question 1246

Topic: 3. Adult Reconstruction (Hip & Knee)

Figures below demonstrate the radiographs obtained from a 35-year-old woman with end-stage debilitating osteoarthritis of the right hip. She is contemplating total hip arthroplasty (THA). She has a history of right hip dysplasia and underwent hip osteotomy as an adolescent. Over the years, nonsurgical treatment, including weight loss, activity modifications, and intra-articular injections, has failed. Her infection work-up reveals laboratory findings within defined limits. A further work-up reveals elevations in serum cobalt and chromium levels and fluid collections surrounding the hip on MRI with MARS. Revision THA is recommended. The most common complication following revision of a failed metal-on-metal hip arthroplasty is

. infection.
. instability.
. loosening.
. periprosthetic fracture.

Correct Answer & Explanation

. infection.

Explanation

THA has proven durable and reliable for pain relief and improving function for patients with end-stage arthritis. Appropriate bearing selection is critical to minimize wear and hip complications. A metal-on-metal articulation is associated with excellent wear rates in vitro. With its capacity to offer a low wear rate with large femoral heads, it is an attractive bearing choice for THA. However, local soft-tissue reactions, pseudotumors, and potential systemic reactions including renal failure, cardiomyopathy, carcinogenesis, and potential teratogenesis with potential transfer of metal ions across the placental barrier make metal-on-metal bearings less desirable and relatively contraindicated for younger women of childbearing age. The workup of a painful metal-on-metal hip arthroplasty necessitates a systematic approach. Several algorithms have been proposed. Routine laboratory studies including sedimentation rate, CRP, and serum cobalt and chromium ion levels should be obtained for all patients with pain. Advanced imaging including MARS MRI should be performed to evaluate for the presence of fluid collections, pseudotumors, and abductor mechanism destruction. Infection can coexist with metal-on-metal reactions, so, when indicated (if the CRP level is elevated), a hip arthrocentesis should be obtained. However, in this setting, a manual cell count and differential should be obtained because an automated cell counter may provide falsely elevated cellcounts. The results of revision surgery for a failed metal-on-metal hip prosthesis can be variable. The amount of local tissue destruction and the integrity of the hip abductor mechanism can greatly influence outcomes. Instability is the most common complication following revision of failed metal-on-metal hip replacements.

Question 1247

Topic: 3. Adult Reconstruction (Hip & Knee)

During revision total knee arthroplasty (TKA), there is significant laxity in 90° of flexion and 10° short of full extension. Correcting the gap imbalance is best achieved by

. resecting more tibia.
. resecting more distal femur to raise the joint line, along with resecting more tibia.
. increasing femoral component size.
. resecting distal femur and increasing femoral component size.When performing revision TKA, the management of gap imbalance and joint line is of critical importance. The flexion gap is generally driven by femoral component size; increasing femoral component size by tightening the flexion gap and downsizing the femoral component size increases the flexion gap. Resecting more distal femur will open up the extension gap; augmenting the femur distally will tighten up the extension gap. Resecting more tibia affects both flexion and extension gaps equally.

Correct Answer & Explanation

. resecting more distal femur to raise the joint line, along with resecting more tibia.

Explanation

Figures 1 through 5 are the radiographs and MRI scans of an 80-year-old woman who had a total hip arthroplasty (THA) 10 years ago and recently experienced an episode of dislocation that was reduced. She currently has no pain, but has a limp and moderate apprehension. Her erythrocyte sedimentation rate is 32 and C-reactive protein is 34. Her cobalt level is 32.8 ug/L (normal <1ug/L) and chromium level 14 ug/L (normal < 5ug/L). The hip aspiration is negative. What is the most appropriate treatment? 35A. Nonoperative treatment with close radiographic follow-upB. Revision THA with ceramic- on-polyethylene with abductor reconstructionC. Removal of components and placement of spacer as stage 1 of 2-stage revisionD. Revision THA with metal-on- polyethylene and trochanteric slide

Question 1248

Topic: 3. Adult Reconstruction (Hip & Knee)
A 38-year-old woman with diabetes mellitus reports a 6-week history of fever and pain localized to the right sternoclavicular joint. Local signs on examination include swelling about the joint, erythema, and increased warmth. Initial aspiration of the joint reveals Staphylococcus aureus. Radiographs reveal medial clavicular osteolysis. What is the most effective treatment at this time?
. Broad-spectrum parenteral antibiotics
. Repeat aspirations
. Irrigation and debridement
. Hyperbaric oxygen
. Resection of the sternoclavicular joint

Correct Answer & Explanation

. Resection of the sternoclavicular joint

Explanation

Based on the findings, the treatment of choice is resection of the sternoclavicular joint. Antibiotic therapy, repeat aspirations, hyperbaric oxygen, and simple irrigation and debridement are generally ineffective and associated with a high rate of recurrence.

Question 1249

Topic: 3. Adult Reconstruction (Hip & Knee)
A follow-up examination of a patient 6 weeks after knee surgery reveals a range of motion from 5° to 55° of flexion. Which of the following statements best summarizes the role of manipulation under anesthesia for this patient?
. Manipulation under anesthesia offers the best chance of improving and maintaining the patient’s range of motion.
. The gains from manipulation under anesthesia are only temporary and rarely last more than 6 months.
. Increasing the frequency and intensity of physical therapy over the next 2 months will have the same effect as manipulation under anesthesia.
. The risks of fracture are so great from manipulating a knee that the patient should be advised to live with a limited range of motion.
. The patient’s final result will be poor with or without manipulation.

Correct Answer & Explanation

. Manipulation under anesthesia offers the best chance of improving and maintaining the patient’s range of motion.

Explanation

DISCUSSION: Esler and associates evaluated the use of manipulation under anesthesia in 47 knees. Manipulation was considered when intensive physical therapy failed to increase flexion to more than 80°. The mean time from arthroplasty to manipulation was 11.3 weeks, and the mean active flexion before manipulation was 62°. One year later, the mean gain was 33°. Definite sustained gains in flexion were achieved even when manipulation was performed 4 or more months after arthroplasty. An additional 21 patients who met the criteria for manipulation declined the procedure, and despite continued physical therapy, they showed no significant increase in knee flexion.

Question 1250

Topic: Total Knee Arthroplasty (TKA)

Radiographs shown in Figures 1 through 3 show two different prosthetic design variations of the same knee implant. When compared with the design of right knee prosthesis, the left can be expected to have a

. higher incidence of patellar clunk and similar implant survivorship.
. higher incidence of patellar clunk and superior implant survivorship.
. lower incidence of patellar clunk and superior implant survivorship.
. lower incidence of patellar clunk and similar implant survivorship.The images show a left posterior stabilized knee prosthesis and a right cruciate sacrificing (ultracongruent / dished type) knee prosthesis. Posterior stabilized designs have a risk of patellar clunk due to the presence of the femoral box with some designs, such as the one shown, exhibiting higher rates. Clinical outcomes are similar between cruciate-retaining, cruciate- sacrificing and posterior stabilized designs.

Correct Answer & Explanation

. higher incidence of patellar clunk and similar implant survivorship.

Explanation

A 76-year-old woman has had three hip revisions for instability. She presents to the emergency department with another dislocation that occurred while getting up from a low chair. Current radiographs are shown in Figures 1 and 2. Her prior AP pelvis radiograph is shown in Figure 3. ESR and CRP are normal. What is the best plan for definitive treatment?

Question 1251

Topic: 3. Adult Reconstruction (Hip & Knee)

Scapular notching following reverse shoulder arthroplasty may be minimized by what technical modification? Review Topic

. Horizontal humeral cut
. Superior inclination of the baseplate
. Inferior inclination of the baseplate
. Use of a 36-mm glenosphere
. Use of a retentive polyethylene liner

Correct Answer & Explanation

. Horizontal humeral cut

Explanation

Biomechanical studies have shown that a 10-degree inferior inclination may decrease scapular notching; whereas superior inclination may worsen notching. Scapular notching has been recognized as a complication following reverse shoulder arthroplasty. Mechanical abutment of the humeral component possibly leads to erosion of the anteroinferior scapular neck, with progressive vulnerability of the inferior baseplate screws. A horizontal humeral cut does not affect notching because the humeral component causes the notching, not the bone on the humerus. Glenosphere size has not been shown to correlate with scapular notching.

Question 1252

Topic: 3. Adult Reconstruction (Hip & Knee)
According to Musculoskeletal Infection Society (MSIS) guidelines, which set of patient laboratory study results fits the definition of chronic prosthetic joint infection?
. ESR 25 mm/hr, CRP 7 mg/L, joint aspiration WBC count 252, 82% neutrophils, and negative leukocyte esterase
. Erythrocyte sedimentation rate (ESR) 50 mm/hr, C-reactive protein (CRP) 8 mg/L, joint aspiration white blood cell (WBC) count 542, 62% neutrophils, and positive leukocyte esterase
. ESR 42 mm/hr, CRP 12 mg/L, joint aspiration WBC count 3,540, 72% neutrophils, and positive leukocyte esterase
. ESR 20 mm/hr, CRP 15 mg/L, joint aspiration WBC count 4,135, 54% neutrophils, and negative leukocyte esterase

Correct Answer & Explanation

. ESR 42 mm/hr, CRP 12 mg/L, joint aspiration WBC count 3,540, 72% neutrophils, and positive leukocyte esterase

Explanation

DISCUSSION: The MSIS definition of periprosthetic joint infection was updated in 2014 with two major and six minor criteria. The presence of one major criterion or three minor criteria is diagnostic for infection. The major criteria are two positive cultures with the same organism or a draining sinus tract. The current MSIS minor criteria are 1) an elevated ESR (more than 30 mm/hr) and CRP level (more than 10 mg/L), 2) an elevated synovial WBC count (more than 3,000 cells per/microliter), 3) an elevated synovial fluid polymorphonuclear count (more than 80%), 4) a positive histological analysis of periprosthetic tissue, and 5) a single positive culture.

Question 1253

Topic: 3. Adult Reconstruction (Hip & Knee)
A 33-year-old female with generalized ligamentous laxity is diagnosed with spontaneous atraumatic subluxation of the sternoclavicular joint. She notes mild, intermittent pain and a small amount of prominence to that area. What is the most appropriate treatment at this time?
. No treatment is indicated at this time
. Figure of eight brace
. Resection arthroplasty of the sternoclavicular joint
. Sternoclavicular and costoclavicular ligament reconstruction
. Sternoclavicular arthrodesis

Correct Answer & Explanation

. No treatment is indicated at this time

Explanation

DISCUSSION: Spontaneous atraumatic subluxation of the sternoclavicular joint is a rare condition and is generally associated with ligamentous laxity. Higginbotham et al reported that spontaneous atraumatic anterior subluxation of the sternoclavicular joint may occur during overhead elevation of the arm. The majority of cases are not painful, and the subluxation usually reduces with lowering the arm. Surgery is rarely indicated. Nonsurgical management, including patient education of the benign nature of the condition, is recommended. Rockwood et al reviewed a series of 37 patients with this condition and noted that at an average follow-up of eight years, the twenty-nine patients who were treated non-operatively had excellent results, with no limitations of activity or changes in lifestyle. The eight patients who were treated operatively (group II) had numerous problems, including noticeable scars, persistent instability, pain, or limitation of activity that resulted in an alteration in lifestyle. The referenced article by Higginbotham is a review of atraumatic disorders of the sternoclavicular joint.

Question 1254

Topic: 3. Adult Reconstruction (Hip & Knee)
A 56-year-old man underwent right total shoulder arthroplasty 2 months ago. Recently while reaching with his shoulder in a flexed and adducted position, he noted shoulder pain and afterwards he could not externally rotate his arm. An axillary radiograph is shown in Figure 30. What is the most likely cause of this problem?
. Excessive anteversion of the humeral component
. Excessive anteversion of the glenoid component
. Excessive posterior capsular laxity
. Anterior capsular laxity
. Use of an oversized humeral head

Correct Answer & Explanation

. Excessive posterior capsular laxity

Explanation

DISCUSSION: Anteversion of the humeral component may result in anterior instability of the component. Posterior instability after total shoulder arthroplasty is usually the result of some combination of the following factors: untreated anterior soft-tissue contractures, excessive posterior capsular laxity, and excessive retroversion of the humeral and/or glenoid components. REFERENCES: Cofield RH, Edgerton BC: Total shoulder arthroplasty: Complications and revision surgery. Instr Course Lect 1990;39:449-462. Wirth MA, Rockwood CA Jr: Complications of total shoulder replacement arthroplasty. J Bone Joint Surg Am 1996;78:603-616.

Question 1255

Topic: 3. Adult Reconstruction (Hip & Knee)
Compared with retention of the native patella in primary total knee arthroplasty, routine patellar resurfacing is associated with
. no patellar complications.
. an increased occurrence of anterior knee pain.
. a reduced patellar fracture rate.
. a reduced risk for revision surgery.

Correct Answer & Explanation

. a reduced risk for revision surgery.

Explanation

DISCUSSION: Despite concerns regarding fracture, osteonecrosis, and patellar clunk, the routine retention of the native patella during primary total knee replacement is associated with a 20% to 30% increased revision risk in large joint registries. In addition, the retention of the native patella results in a 5.7% revision surgery rate in patients with anterior knee pain.

Question 1256

Topic: 3. Adult Reconstruction (Hip & Knee)
Figure 33 shows the AP and lateral radiographs of an obese 58-year-old man who underwent a cementless total hip arthroplasty 6 years ago. He reports no pain, and examination reveals a normal gait and painless hip range of motion. What is the most likely diagnosis?
. Osteolysis because of polyethylene debris
. Spot weld
. Osteosclerotic pedestal
. Cement fragmentation
. Allergic reaction to titanium

Correct Answer & Explanation

. Osteolysis because of polyethylene debris

Explanation

Osteolysis of an otherwise well-functioning total hip arthroplasty is a recognized complication, and its radiographic appearance is typical, as shown here. Distal osteolysis, such as that shown here, is more prevalent when there is noncircumferential sealing of the proximal femoral canal.

Question 1257

Topic: Total Hip Arthroplasty (THA)
During a revision total hip arthroplasty, the surgeon encounters a severe acetabular defect. Radiographs and intraoperative findings reveal greater than 3 cm of superior component migration, destruction of the teardrop, and component medialization beyond Kohler's line. The superior hemipelvis remains mechanically in continuity with the inferior hemipelvis. According to the Paprosky classification, what is the defect grade and the most appropriate reconstructive option?
. Paprosky IIIA; hemispherical jumbo cup with multiple screws
. Paprosky IIIB; custom triflange or cup-cage construct
. Paprosky IIIC; massive structural allograft with a cemented cup
. Paprosky IVA; pelvic discontinuity requiring distraction and plating
. Paprosky IIC; highly porous titanium cup with augment

Correct Answer & Explanation

. Paprosky IIIB; custom triflange or cup-cage construct

Explanation

A Paprosky IIIB defect is characterized by severe bone loss with >3 cm of superior migration, component medialization past Kohler's line, and destruction of the teardrop, indicating non-supportive superior and medial bone (<30% host bone contact). Management typically requires a custom triflange component, a cup-cage construct, or massive structural allografts with a support ring to achieve stability, as standard hemispherical cups will not achieve adequate fixation.

Question 1258

Topic: 3. Adult Reconstruction (Hip & Knee)

During a primary total hip arthroplasty, the surgeon intentionally medializes the acetabular component to the level of the true floor of the acetabulum. Which of the following best describes the biomechanical consequence of this maneuver?

. Increases the body weight lever arm and increases the joint reaction force
. Decreases the abductor lever arm and increases the joint reaction force
. Decreases the body weight lever arm and decreases the joint reaction force
. Increases the abductor lever arm and increases the body weight lever arm
. Shifts the center of rotation superiorly and increases the joint reaction force

Correct Answer & Explanation

. Increases the body weight lever arm and increases the joint reaction force

Explanation

Medializing the center of rotation of the hip decreases the body weight lever arm (the distance from the body's center of gravity to the center of rotation of the hip). By reducing the body weight lever arm, the force required by the abductors to maintain a level pelvis is reduced, which consequently decreases the overall joint reaction force across the hip. This is an essential principle in reconstructing hip biomechanics.

Question 1259

Topic: 3. Adult Reconstruction (Hip & Knee)

A 62-year-old male with a metal-on-polyethylene total hip arthroplasty presents with new-onset groin pain and a palpable mass 6 years postoperatively. Radiographs show a well-fixed implant with no osteolysis. Serum cobalt levels are markedly elevated, while chromium levels are normal. MARS MRI reveals a large cystic mass communicating with the joint. Which of the following implant characteristics is most strongly associated with this complication?

. Small diameter (28 mm) cobalt-chromium femoral head
. Use of a highly cross-linked polyethylene liner
. Large diameter (36 mm or larger) cobalt-chromium femoral head on a titanium stem
. A ceramic femoral head on a titanium stem
. A fully porous-coated cobalt-chromium femoral stem

Correct Answer & Explanation

. Small diameter (28 mm) cobalt-chromium femoral head

Explanation

The clinical presentation describes Adverse Local Tissue Reaction (ALTR) secondary to mechanically assisted crevice corrosion (MACC) at the head-neck junction (trunnionosis). This is characteristically associated with elevated serum cobalt levels out of proportion to chromium. Risk factors include larger diameter cobalt-chromium heads (which increase torque at the trunnion), high-offset necks, and a dissimilar metal pairing (e.g., CoCr head on a titanium stem).

Question 1260

Topic: Total Hip Arthroplasty (THA)

A surgeon is considering using a dual mobility articulation for an 80-year-old patient undergoing revision THA for recurrent instability due to severe abductor deficiency. Which of the following best describes the biomechanical mechanism by which a dual mobility implant increases stability?

. It decreases the head-to-neck ratio, allowing increased range of motion before impingement
. It increases the 'jumping distance' through the use of a large effective head size consisting of a mobile polyethylene liner
. It constrains the femoral head within the liner using a locking ring mechanism
. It functions by lateralizing the center of rotation to tension the remaining soft tissues
. It exclusively relies on a bipolar design where motion occurs only at the inner interface

Correct Answer & Explanation

. It decreases the head-to-neck ratio, allowing increased range of motion before impingement

Explanation

Dual mobility implants feature a smaller inner metallic or ceramic head that articulates within a larger, mobile polyethylene sphere, which in turn articulates within a metal acetabular shell. The primary mechanism of increased stability is the large effective head size of the outer polyethylene sphere, which significantly increases the 'jumping distance' (the distance the head must translate to dislocate) and maximizes the impingement-free range of motion.

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