Femoral head sizes larger than 32 mm against highly cross-linked polyethylene

Developing Patient-Specific Statistical Reconstructions of Healthy Anatomical Structures to Improve Patient Outcomes

There are still numerous problems with modern joint replacement prostheses, which negatively influence patient health and recovery. For example, it is especially important to avoid failures and complications following hip arthroplasty because the loss of hip joint function is commonly associated with increased demand on the healthcare system, reoperation, loss of independence, physical disability, and death. The current study uses hip arthroplasty as a model system to present a new strategy of computationally generating patient-specific statistical reconstructions of complete healthy anatomical structures from computed tomography (CT) scans of damaged anatomical structures. The 3D model morphological data were evaluated from damaged femurs repaired with prosthetic devices and the respective damaged femurs that had been restored using statistical reconstruction. The results from all morphological measurements (i.e., maximum femoral length, Hausdorff distance, femoral neck anteversion, length of rotational center divergence, and angle of inclination) indicated that the values of femurs repaired with traditional prostheses did not fall within the +/-3 standard deviations of the respective patient-specific healthy anatomical structures. These results demonstrate that there are quantitative differences in the morphology of femurs repaired with traditional prostheses and the morphology of patient-specific statistical reconstructions. This approach of generating patient-specific statistical reconstructions of healthy anatomical structures might help to inform prosthetic designs so that new prostheses more closely resemble natural healthy morphology and preserve biomechanical function. Additionally, the patient-specific statistical reconstructions of healthy anatomical structures might be valuable for surgeons in that prosthetic devices could be selected and positioned to more accurately restore natural biomechanical function. All in all, this contribution establishes the novel approach of generating patient-specific statistical reconstructions of healthy anatomical structures from the CT scans of individuals' damaged anatomical structures to improve treatments and patient outcomes.

Superior functional outcome with dual mobility THR as compared to conventional THR in fracture neck femur: a prospective cohort study

Introduction: Total Hip Replacement (THR) in displaced Fracture Neck of Femur (FNOF) is associated with higher dislocation rates. Conventional THR with a large femoral head and anterior approach has reduced the instability, but it remains higher than THR done for other aetiology. Recent studies have shown reduced dislocation rates with dual mobility THR (DMTHR) for FNOF; however, there is a lack of comparative research to show its superiority over conventional THR. Further, its role in the Asian subcontinent, where the patient requires sitting cross-legged or squatting, has not been studied. Methods: A prospective cohort study of 103 elderly patients with displaced FNOF with a minimum follow-up of 1-year. Fifty-two patients were operated on with DMTHR and fifty-one patients with conventional THR. Both the groups were matched in terms of demographic data, surgical approach, and postoperative protocol. Radiological and functional outcomes in terms of Harris Hip Score (HHS), Range of motion, Patient Reported Outcome Measures (PROM), and Dislocation rate were compared between the two groups. Results: Mean HHS of the DMTHR group was 76.37 at three months and 87.02 at the end of the 1-year postoperatively, which was significantly better than the conventional THR group 65.65 at three months and 72.96 at 1-year. The range of motion was significantly better in the DMTHR group than the conventional THR group. There was no significant difference in radiological outcomes and postoperative dislocation rate between the two groups. Conclusion: Dual mobility implants give better results than conventional implants for primary THA in elderly patients of displaced FNOF regarding better function and greater range of motion.

Prospective Study Showing Results of Large-Diameter Femoral Heads After Cementless Total Hip Replacement

Introduction

Large-diameter femoral heads (≥36 mm) were introduced to decrease instability and improve the range of motion of the hip. We hypothesized regarding the clinical outcome and complications (dislocation, implant survivorship, and functional scores) following total hip replacement (THR) surgery in an Indian population who have smaller acetabulum compared to the western population.

Methodology

A prospective study was conducted at a tertiary hospital from November 2011 to July 2013. A total of 70 patients with hip pathology were operated by a senior surgeon for THR using the anterolateral approach. The Harris Hip Scores were recorded pre and postoperatively in all patients. Postoperatively, radiographs were taken to check for evidence of implant loosening or osteolysis. The patients were followed up till a mean follow-up of 86.52 months (range: 74 to 108 months) in our cohort. Detailed clinical and radiographic results were available for 59 patients, while six died (three died of myocardial infarction (MI), two had cerebrovascular accident (CVA), and one patient died of pulmonary embolism) and five patients were lost to follow-up.

Results

Of the 59 hips, majority (76%) had acetabular inclination of 46-55 degrees. Ninety percent of the stems were in the central position and 10% were in the varus position. The average preoperative Harris Hip Score was 38.8 ± 5.7 (range: 24-46), which increased to 90.4 ± 7.3 (range: 78-94) at the last follow-up. A total of six patients died (four died of MI and two of CVA) and two patients had infection which was treated with antibiotics. Three cases of dislocation were observed; one following a fall one year after surgery and revision total hip arthroplasty was done and two cases while getting up from the bed which were managed with closed reduction and abduction brace for six weeks. Two cases of periprosthetic fracture were observed which were managed with plating.

Conclusion

Lower dislocation rate and better range of movement reinforces the advantage of large-diameter femoral head during THR in the Indian population.

Dual mobility THR: Resolving instability and providing near normal range of movement

BACKGROUND

Dislocation is a common complication of Total Hip Replacement (THR), particularly when performed in primary (indications with increased risk of instability) and in revision scenarios. Dual mobility THR (DMTHR) minimizes the risk of instability in such scenarios, however most of the evidence is from western literature. Results of DMTHR in Indian scenarios where patient want to go back to their normal routine activities of squatting and sitting cross-legged is lacking. The aim of our study was to evaluate the short to mid-term results of DMTHR for varied indications (both primary and revision) in Indian scenario. To evaluate the outcome of the DMTHR in terms of functional range of motion and the ability to go back to their pre-injury level of activity.

METHODS

This is a retrospective study of 150 patients operated with DMTHR between January 2015 to February 2019 with a minimum follow-up of 12 months. Patients were evaluated clinically using Modified Harris Hip Score (HHS), Range of Motion (ROM), and Patient Reported Outcome Measures (PROM) like ability to squat and sit cross legged. Radiological evaluation was done using radiographs to assess loosening, stress shielding, osteolysis.

RESULTS

Mean follow up in our study was 25.2 months (Range 12-46 months). Mean Modified HHS was 71.8 ± 8.11 at 6 weeks post-op and 85.8 ± 7.62 at last follow-up. HHS showed excellent outcome in 36 hips (26.7%), good outcome in 76 hips (56.7%), fair outcome in 20 hips (14.6%), poor outcome in 3 hips (2%). All our patients were allowed to squat and sit cross-legged at a mean follow-up period of 13 weeks (8 weeks-20 weeks) except 10 cases of Revision THR where patients were advised not to squat or sit cross-legged. All patients were able to resume their activities of daily living.

CONCLUSION

DMTHR in patients of all ages has shown a good short to midterm clinical outcome which is comparable to conventional THR. It confers the benefit of stability allowing our patients to squat and sit cross legged which is often one of the expectation and requirement of a patient undergoing THR in India. DMTHR in both primary and revision scenarios exhibit a low risk of dislocation, complications and revision surgery.

Clinical and radiological results with a 36-mm cobalt-chrome prosthetic head, cross-linked Durasul liners associated with Allofit cups: a more than 10-year follow-up period

BACKGROUND

Highly cross-linked polyethylene is currently a common articulation surface used for total hip arthroplasty (THA).

AIM

The aim of the present study is in vivo assessment of highly cross-linked Durasul polyethylene linear and volumetric wear when associated with a 36-mm prosthetic femoral head.

METHODS

We retrospectively reviewed clinical and radiographic data of 78 patients (81 hips) having primary THAs using Durasul liner combined with a 36-mm CoCr prosthetic head. All of them were followed for more than 10 years. Patient outcome was assessed with the Harris Hip Score (HHS) preoperatively and at last follow-up. 2-D prosthetic head penetration into polyethylene, 3D wear rates and cup migration were evaluated.

RESULTS

The preoperative and last follow-up HHS were 50.43 +/- 10.42 and 97.44 +/- 5.51 respectively. The annual penetration of the prosthetic head into Durasul^® liner was 0.029 +/- 0.003 mm. The annual linear penetration and volumetric wear extrapolation rates using Charnley and Ilchmann formulas were 37.84% and 57.76% respectively of that seen with conventional polyethylene liner. At last follow-up, the total loss of material in Durasul represents only 0.15% of the initial polyethylene mass. We did not observe any significant cup migration in the study group.

CONCLUSIONS

Results are promising, and we believe that these data authorise the continued use of highly cross-linked polyethylene liner associated with a 36-mm prosthetic head for total hip arthroplasties in older patients. More long-term follow-up studies are mandatory before we feel comfortable with the project of using cross-linked polyethylene in young and active patients instead of ceramic-on-ceramic bearings.

Neer Award 2017: wear rates of 32-mm and 40-mm glenospheres in a reverse total shoulder arthroplasty wear simulation model

BACKGROUND

Larger glenosphere diameters have been used recently to increase prosthesis stability and impingement-free range of motion in reverse total shoulder arthroplasty. The goal of this study was to evaluate the rate of polyethylene wear for 32-mm and 40-mm glenospheres.

METHODS

Glenospheres (32 mm and 40 mm, n = 6/group) and conventional polyethylene humeral liners underwent a 5-million cycle (MC) wear simulation protocol. Abduction-adduction and flexion-extension motion profiles were alternated every 250,000 cycles. At each interval, mass loss was determined and converted to volume loss and wear rate. At 0, 2.5 MC, and 5 MC, liners were imaged using micro-computed tomography to determine surface deviation. White light interferometry was performed on liners and glenospheres at 0 and 5 MC to quantify surface roughness. Wear particle morphology was characterized by environmental scanning electron microscopy.

RESULTS

Total volume loss was significantly higher in 40-mm liners from 1.5 MC onward (P < .05). Overall, volumetric wear rate was significantly higher in 40-mm liners compared with 32-mm glenospheres (81.7 ± 23.9 mm³/MC vs. 68.0 ± 18.9 mm³/MC; P < .001). However, micro-computed tomography surface deviation results demonstrated increased linear penetration on 32-mm glenospheres compared with 40-mm glenospheres (0.36 ± 0.03 µm vs. 0.28 ± 0.01 µm; P = .002). Surface roughness measurements showed no difference for liners; however, increased roughness was noted for 40-mm glenospheres at 5 MC compared with 32 mm (P < .05).

CONCLUSION

Larger glenospheres underwent significantly greater polyethylene volume loss and volumetric wear rates, whereas smaller glenospheres underwent greater polyethylene surface deviations. The enhanced stability provided by larger glenospheres must be weighed against the potential for increased polyethylene wear.

Wear Rates of Larger-Diameter Cross-Linked Polyethylene at 5 to 13 Years: Does Liner Thickness or Component Position Matter?

BACKGROUND

Cross-linked polyethylene (XLPE) has demonstrated significantly reduced wear and osteolysis into the second decade for total hip arthroplasty. There is a relative paucity of data with ≥36-mm bearings. Issues include potential effects of reduced liner thickness and component position on wear, osteolysis, and mechanical failure of the bearing.

METHODS

Radiographs of 48 primary total hip arthroplasties with ≥36-mm modular XLPE bearings were analyzed at a minimum 5 years postoperative on serial radiographs using a validated, edge-detection-based algorithm. Subgroups were examined to assess the effect of bearing diameter, liner thickness, acetabular abduction angle, and acetabular anteversion on XLPE wear.

RESULTS

There was no significant difference in volumetric wear when subgroups were stratified by component factors: liner thickness (<6.5 mm vs ≥6.5 mm) 40.69 mm³/y vs 24.47 mm³/y, respectively (P = .315); acetabular component abduction angle (<45° vs ≥45°): 38.68 mm³/y vs 27.8 mm³/y, respectively (P = .522); acetabular anteversion (<20° vs ≥20°): 41.32 mm³/y vs 31.79 mm³/y, respectively (P = .521). There were no dislocations, mechanical failures, or revisions. There were 7 hips with volumetric wear rates ≥80 mm³/y; 1 had possible osteolysis.

CONCLUSION

Larger-diameter XLPE wear was not measurably affected by liner thickness, acetabular abduction angle, or acetabular anteversion. However, there is a trend for increasing volumetric wear with increasing bearing size. Wear outliers do occur, and continued follow-up of larger-diameter XLPE bearings is warranted.

Intraoperative evaluation of the effects of femoral component offset and head size on joint stability in total hip arthroplasty

PURPOSE

To evaluate intraoperatively the effects of femoral offset and head size on stability of the hip joints in total hip arthroplasty (THA) via posterior approach.

METHODS

Thirty cementless THAs were included in this study. After acetabular shell and femoral broach fixation, trial reduction was repeated using a femoral neck (5- to 8-mm higher or standard offset) and a head (26 mm or 32 mm). To evaluate joint stability, range of internal rotation (IR) in hip flexion prior to posterior subluxation and range of external rotation (ER) in hip extension were measured.

RESULTS

The high-offset neck provided significantly (approximately 10°) greater range of IR to subluxation than the standard-offset neck. No hips ended in anterior subluxation by ER. The head size did not have significant effects on the stability.

CONCLUSION

Results suggest that the 5- to 8-mm greater femoral offset might be effective in preventing instability in primary THA.

Thirteen-Year Evaluation of Highly Cross-Linked Polyethylene Articulating With Either 28-mm or 36-mm Femoral Heads Using Radiostereometric Analysis and Computerized Tomography

BACKGROUND

The objective of this 13-year prospective evaluation of highly cross-linked ultra high molecular weight polyethylene (HXLPE) was to (1) assess the long-term wear of HXLPE articulating with 2 femoral head sizes using radiostereometric analysis (RSA) and to (2) determine if osteolysis is a concern with this material through the use of plain radiographs and computerized tomography (CT).

METHODS

All patients received a Longevity HXLPE liner with tantalum beads and either a 28-mm or 36-mm femoral head. Twelve patients (6 in each head size group) agreed to return for 13-year RSA, plain radiograph, and CT follow-up. The 1-year and 13-year plain radiographs as well as the CT scans were analyzed for the presence of osteolysis.

RESULTS

The 13-year mean ± standard error steady-state wear was 0.05 ± 0.02 mm with no significant increase over time or between the 2 head size groups. Two patients' CT scans showed radiolucent regions in the acetabulum of 4.51 cm(3) and 11.25 cm(3), respectively. In one patient, this area corresponded to a partially healed degenerative cyst treated with autograft during surgery. The second patient had an acetabular protrusio treated with autograft, and the CT scan revealed areas of remodeling of this graft. One patient's 13-year plain radiographs showed evidence of cup loosening and linear radiolucencies in zones 2 and 3.

CONCLUSION

There was no evidence of significant wear over time using RSA. The CT scans did not show evidence of osteolysis due to wear particles. These results suggest that this material has reduced wear compared to conventional polyethylene, irrespective of head size.

Dislocation rates following primary total hip arthroplasty have plateaued in the Medicare population

The purpose of this study was to determine if the use of larger femoral head diameters, in combination with recent practice including enhanced soft tissue choices and various operative exposure choices has led to any further decline in dislocation rates. 51,901 patients undergoing primary THA were identified from 5% Medicare Part B (physician/carrier) claims between January 1, 1997 and December 31, 2011. Dislocation rate at 6 months following THA was 2.84% over the study period (1997-2011). From 2005 to 2011, dislocation rates following primary THA have plateaued in the United States at approximately 2%. This suggests that the full benefits using large femoral head sizes are now realized. For further improvement in dislocation rates, a greater emphasis will be required on patient selection, surgical technique and component alignment.

Results of Primary Total Hip Arthroplasty with 36-mm Femoral Heads on Highly Cross-linked Polyethylene-Minimum Seven-years Follow-up

Purpose

We evaluate the clinical and radiographic midterm results of primary total hip arthroplasty (THA) using a 36 mm diameter femoral head on highly cross-linked polyethylene (minimum 7-year follow-up).

Materials and Methods

We retrospectively reviewed 73 patients (74 hips) that underwent primary THA with a 36 mm diameter femoral head on highly cross-linked polyethylene between July 2004 and February 2007. Clinical follow-ups included specific measurements like modified Harris hip scores (HHS) and Merle d'Aubigne and Postel score. For radiologic evaluations, together with position of acetabular cup at 6 weeks later of post-operation, we separately calculated the penentrations of femoral heads into polyethylene liners during post-operation and one year later check-ups, and during one year later check-ups and final check-ups.

Results

There were no complications except for one case of dislocation. Average modified HHS at final follow-up was 88±7.5 (range, 81-96), and Merle d'Aubigne and Postel scores were more than 15 (range, 15-18). Mean acetabular cup inclination and anteversion were 50.1°(range, 35°-58°) and 23.6°(range, 5°-38°), respectively. Average femoral head penetration during the first postoperative year was 0.071±0.034 mm/year, and steady-state wear rate determined using radiographs taken at one-year postoperatively and at latest follow-up was 0.051±0.022 mm/year. Average femoral head penetration during entire follow-ups was 0.058±0.013 mm/year.

Conclusion

Primary THA with a large diameter femoral head on highly cross-linked polyethylene was found to produce the results comparable to previous in vitro laboratory hip simulation studies. And we also find out good scores in terms of patient's functionality.

Comparison between 28 mm and 32 mm ceramic-on-ceramic bearings in total hip replacement

Large femoral heads have become popular in total hip replacement (THR) as a method of reducing the risk of dislocation. However, if large heads are used in ceramic-on-ceramic THR, the liner must be thinner, which may increase the risk of fracture. To compare the rates of ceramic fracture and dislocation between 28 mm and 32 mm ceramic heads, 120 hips in 109 patients (51 men and 58 women, mean age 49.2 years) were randomised to THR with either a 28 mm or a 32 mm ceramic articulation. A total of 57/60 hips assigned to the 28 mm group and 55/60 hips assigned to the 32 mm group were followed for at least five years. No ceramic component fractures occured in any patient in either group. There was one dislocation in the 32 mm group and none in the 28 mm group (p = 0.464). No hip had detectable wear, focal osteolysis or prosthetic loosening. In our small study the 32 mm ceramic articulation appeared to be safe in terms of ceramic liner fracture.

Cite this article: Bone Joint J 2014;96-B:1459–63.

Alternative bearing surfaces in total hip arthroplasty

Total hip arthroplasty continues to be one of the most effective surgical procedures currently available. The longevity of this otherwise very successful procedure is compromised by the wear of the bearing surface. In recent years, great advances have been made in the design of bearing surfaces that poise to offer extended performance to these artificial joints. The purpose of this review is to discuss the recent technologic advances in bearing surfaces and offer an insight to the potential benefits and concerns with the alternative bearing surfaces.

Low early and late dislocation rates with 36- and 40-mm heads in patients at high risk for dislocation

BACKGROUND

Large (36- and 40-mm) femoral heads with highly crosslinked polyethylene liners were introduced to reduce the risk of dislocation after primary total hip arthroplasty (THA), but it is unclear whether the risk is reduced and whether there is osteolysis or liner fracture.

QUESTIONS/PURPOSES

We therefore determined (1) the incidence of early and late (> 5 years) dislocation; (2) the rate of femoral and acetabular component loosening and revision; and (3) the rate of liner fracture and pelvic osteolysis.

METHODS

We retrospectively reviewed 112 patients presumed at high risk for dislocation who had 122 primary THAs: 108 with 36-mm and 14 with 40-mm femoral heads. The risk factors were: age > 75 years (80 hips); proximal femur fracture (18); history of contralateral dislocation (two); history of alcohol abuse (two); large acetabulum > 60 mm (six); and other (14). Patients were evaluated for early (< 1 year) and late (> 5 years) dislocation; rate of reoperation; clinical result with Harris hip score; and standard radiographic analysis for radiolucent lines and osteolysis.

RESULTS

The rate of early dislocation was 4% (five of 122 hips), all with a 36-mm head. There were no late dislocations in 74 hips followed for 5 to 10 years, no revision for acetabular or femoral loosening, and no liner fractured. There were no hips with pelvic osteolysis and seven hips with an acetabular radiolucent line.

CONCLUSIONS

The 36- and 40-mm femoral heads were associated with a low risk of dislocation in high-risk patients undergoing primary THA with no osteolysis or liner fracture.

LEVEL OF EVIDENCE

Level IV, therapeutic study. See the Guidelines for Authors for a complete description of level of evidence.

The 2012 John Charnley Award: Clinical multicenter studies of the wear performance of highly crosslinked remelted polyethylene in THA

BACKGROUND

Highly crosslinked polyethylene (HXLPE) in THA was developed to reduce particle-induced periprosthetic osteolysis. A series of clinical studies were initiated to determine the clinical efficacy as judged by patient-reported scores, radiographic osteolysis, and wear analysis of one form of HXLPE.

QUESTIONS/PURPOSES

The purposes of this series of studies were to (1) determine the wear rates of one form of HXLPE; (2) report long-term (7-10 years) patient-reported outcome measures; (3) assess the effect of femoral head size on wear; and (4) determine the incidence of periprosthetic osteolysis.

METHODS

A single-center and two multicenter studies were conducted on 768 primary patients (head size 26-36 mm) undergoing THA at eight medical centers. Patient-reported outcome scores, radiographic grading for osteolysis, and radiographic wear evaluation were performed.

RESULTS

Serial plain radiographs showed no periprosthetic osteolysis in the three studies. The average femoral head penetration rates did not correlate with time in vivo for patients with standard femoral head sizes. Although there was an indication of higher wear in patients with 36-mm diameter femoral heads, it was below the threshold for producing osteolysis.

CONCLUSIONS

The introduction of this HXLPE substantially improved the prognosis of patients after THA up to 13 years as judged by clinical scores, incidence of osteolysis, and polyethylene wear measurements.

LEVEL OF EVIDENCE

Level III, therapeutic study. See the Guideline for Authors for a complete description of levels of evidence.

What factors affect posterior dislocation distance in THA?

BACKGROUND

Dislocation remains common after total hip arthroplasty. Efforts have been made to identify and minimize risk factors. One such factor, jump distance, or the distance the femoral head must travel before dislocating, has been poorly characterized with respect to three-dimensional kinematics.

QUESTIONS/PURPOSES

We therefore determined: (1) the three-dimensional stability of four different component designs; (2) whether the degree of abduction and anteversion affects the stability; (3) whether pelvic inclination angles affected stability; and (4) which combination of these three factors had the greatest stability.

METHODS

We created a positionable three-dimensional model of a THA. Acetabular components were modeled in various abduction and anteversion angles and in two different pelvic inclinations which simulate standing and chair-rising activities.

RESULTS

The posterior horizontal dislocation distance increased as inclination angle and femoral head size increased. The 48-mm resurfacing typically had lower jump distances and was at risk of posterior edge loading at 30° inclination. The highest jump distance for all positions and activities occurred with the dual-mobility bearing.

CONCLUSION

These findings suggest that monoblock cups require extremely accurate positioning for low dislocation risk and that pelvic orientation may increase dislocation risks.

CLINICAL RELEVANCE

As a result of the dual-mobility designs having the greatest resistance to dislocation, these cups may be appropriate for patients who are at risk for dislocation in difficult primary situations and in revision hip arthroplasty procedures in which proper component orientation may be less likely to be achieved.

The Frank Stinchfield Award: Dislocation in revision THA: do large heads (36 and 40 mm) result in reduced dislocation rates in a randomized clinical trial?

BACKGROUND

Dislocation after revision THA is a common complication. Large heads have the potential to decrease dislocation rate, but it is unclear whether they do so in revision THA.

QUESTIONS/PURPOSES

We therefore determined whether a large femoral head (36 and 40 mm) resulted in a decreased dislocation rate compared to a standard head (32 mm).

METHODS

We randomized 184 patients undergoing revision THA to receive either a 32-mm head (92 patients) or 36- and 40-mm head (92 patients) and stratified patients by surgeon. The two groups had similar baseline demographics. The primary end point was dislocation. Quality-of-life (QOL) measures were WOMAC and SF-36. The mean followup for dislocation was 5 years (range, 2-7 years); the mean followup for QOL was 2.2 years (range, 1.6-4 years).

RESULTS

In the 36- and 40-mm head group, the dislocation rate was 1.1% (one of 92) versus 8.7% (eight of 92) for the 32-mm head. There was no difference in QOL outcomes between the two groups.

CONCLUSIONS

Our observations confirm a large femoral head (36 or 40 mm) reduces dislocation rates in patients undergoing revision THA at short-term followup. We now routinely use large heads with a highly crosslinked polyethylene acetabular liner in all revision THAs.

LEVEL OF EVIDENCE

Level I, therapeutic study. See Guidelines for Authors for a complete description of levels of evidence.

Results after total hip arthroplasty with a large head and bipolar arthroplasty in patients with displaced femoral neck fractures

The authors studied the short-term outcomes of total hip arthroplasty (THA) performed using large diameter femoral heads or bipolar arthroplasty (BA) in physiologically active elderly patients with displaced intracapsular femoral neck fractures. The THA group included 14 males and 66 females with a mean age of 75.5 years, and the BA group included 16 males and 73 females with a mean age of 77.6 years. Surgical procedures were performed by one surgeon using a modified Hardinge approach. Mean operation times were significantly longer in the THA groups. Pain, mobility, and walking ability scores were significantly better in the THA group than in the BA group. Despite no range of motion limitation during the early postoperative period, no dislocation was encountered in either group. The present study suggests that for displaced femoral neck fractures, THA with a large diameter femoral head results in less pain and better function than BA.

Do large heads enhance stability and restore native anatomy in primary total hip arthroplasty?

BACKGROUND

Dislocation remains a serious complication in hip arthroplasty. Resurfacing proponents tout anatomic femoral head restoration as an advantage over total hip arthroplasty. However, advances in bearings have expanded prosthetic head options from traditional sizes of 22, 26, 28, and 32 mm to diameters as large as 60 mm. Large heads reportedly enhance stability owing to increased range of motion before impingement and increased jump distance to subluxation. Available larger diameter material combinations include metal- or ceramic-on-highly crosslinked polyethylene and metal-on-metal, each with distinct advantages and disadvantages.

QUESTIONS/PURPOSES

We sought to determine (1) if using larger diameter heads has lowered our dislocation rate; and (2) how closely an anatomic metal-on-metal bearing with diameters to 60 mm replicates native femoral head size.

METHODS

We retrospectively reviewed 2020 primary arthroplasties performed with large heads (≥ 36 mm) in 1748 patients and noted dislocation incidence. In a prospective subset of 89 cases using anatomic heads, native femoral head diameter was measured intraoperatively with calipers by an independent observer and later compared with implanted size.

RESULTS

One dislocation has occurred in 2020 hips for an incidence of 0.05%. The prosthetic head averaged 0.7 mm larger than the native head with 68 of 89 (76%) reconstructed to within ± 2 mm of native size.

CONCLUSIONS

Larger diameter heads have contributed to lower dislocation rates and large-diameter metal-on-metal articulation can provide close anatomic restoration in primary THA.

Femoral head size does not affect ion values in metal-on-metal total hips

BACKGROUND

Metal-on-metal articulations can release substantial amounts of particles containing cobalt and chromium into the surrounding milieu, causing concern for cellular toxicity and adverse local soft tissue reactions. The diameter of the femoral head has been one of the variables that inversely affects wear of metal-on-metal total hip arthroplasty (THA). The oxidative stress of increased metal ions can be measured with serum markers. It is still controversial if larger femoral head diameters decrease wear rates in patients with metal-on-metal THA and if the increased metal ions alter the body's antioxidant status.

QUESTIONS/PURPOSES

We therefore (1) determined whole blood metal ions in patients with small (28 mm and 36 mm) and large (40 mm and 44 mm) diameter femoral heads; (2) measured oxidative stress markers (total antioxidants, nitrotyrosine, and peroxides); and (3) determined whether acetabular version or inclination influenced ion levels.

METHODS

One hundred four patients were retrospectively studied. We recorded Harris hip scores and UCLA activity scores. All patients were followed at 1 year.

RESULTS

The activity scores were similar in the two groups. There was no difference in metal ion levels or oxidative stress markers between patients with small- or large-diameter femoral heads. Acetabular inclination and anteversion had no effect on the metal ion levels.

CONCLUSIONS

The data suggest there is no difference in ion values in patients with large or small metal-on-metal THA and the increased metal ions do not alter the oxidant status of the patient.

Wear and range of motion of different femoral head sizes

Femoral head sizes greater than 32 mm are more prevalent with current total hip arthroplasty. We hypothesized that linear wear rates of Durasul highly cross-linked polyethylene would not differ with different head sizes. We also compared the range of motion of the hip. Ninety-four consecutive arthroplasties in 84 patients were studied for a mean 3.6 +/- 0.7 years. There was no statistical difference in linear wear rates and annual or total penetration rates when 28-mm and 32-mm heads were compared to 38-mm and 44-mm heads. Volumetric wear was 12.4 mm(3)/y higher with bigger heads. Range of motion did not differ. Larger femoral head sizes show no evidence of an accelerated wear pattern when used with Durasul.

The effects of acetabular shell deformation and liner thickness on frictional torque in ultrahigh-molecular-weight polyethylene acetabular bearings

The purposes of this study were to determine if there were differences in the frictional torque generated between spherical acetabular shells and acetabular shells deformed as a result of implantation and to evaluate how changes in polyethylene insert thickness and head diameter affected these frictional torque data. An established bench top model was used for mechanical testing. A total of 70 samples were tested. Acetabular shells were impacted into polyurethane foam that was designed to create spherical or deformed shell models. We found that deformed acetabular shells produced higher frictional torque than spherical shells. Also, larger femoral head sizes produced greater frictional torque than smaller femoral head sizes. For the deformed models, the thicker polyethylene inserts produced greater frictional torque than the thinner polyethylene inserts.

High stress conditions do not increase wear of thin highly crosslinked UHMWPE

Introduction of highly crosslinked polyethylene has increased interest in large femoral heads, because thin acetabular liners can be used while maintaining low wear rates and larger heads decrease the incidence of instability. However, crosslinking and subsequent thermal treatments can cause decreased mechanical properties that might obviate the reduced wear under extreme conditions. To examine whether increased contact pressures would adversely affect wear in thin liners, we tested thin and thick highly crosslinked liners (3.8 mm thickness/44-mm head and 7.9 mm thickness/36-mm head, respectively) to 5 million cycles on a hip simulator under near impingement conditions. Conventional polyethylene liners (7.9 mm thickness/36-mm head) served as controls. Large femoral heads with highly crosslinked polyethylene liners as thin as 3.8 mm in thickness do not wear at a higher rate than a thicker liner of the same material, even when subjected to large contact pressures such as occur under near-impingement conditions. Crosslinked polyethylene may allow for liners that are thinner than has been traditionally accepted. This conclusion, however, is based solely on wear test results with idealized cup position, no intentional edge loading, no head subluxation, and no artificial aging. Continued monitoring will be necessary to elucidate the clinical efficacy of these devices.

Femoral head size and wear of highly cross-linked polyethylene at 5 to 8 years

Wear of highly cross-linked polyethylene is reportedly independent of head size. To confirm that observation we asked in our population whether head size related to wear with one type of electron beam highly cross-linked polyethylene. Of 146 hips implanted, we evaluated complete clinical and radiographic data for 90 patients (102 hips or 70%). The minimum followup was 5 years (mean, 5.7 years; range, 5-8 years). The head size was selected intraoperatively based on the size of the acetabular component and presumed risk of dislocation. Polyethylene wear measurements were performed in one experienced laboratory using the method of Martell et al. There was no hip with pelvic or femoral osteolysis. The median linear wear rate was 0.028 mm/year (mean, 0.04 mm/year), and the median volumetric wear rate was 25.6 mm(3)/year (mean, 80.5 mm(3)/year). Median total volumetric wear was 41.0 mm(3) (mean, 98.5 mm(3)). We found no association between femoral head size and the linear wear rate, but observed an association between larger (36- and 40-mm) head size and volumetric wear rate and total volumetric wear. Although the linear wear rate of polyethylene was not related to femoral head diameter, there was greater volumetric wear (156.6 mm(3)/year) with the 36- and 40-mm heads. Pending long-term studies of large head sizes, we advise caution in using larger femoral heads in young or active patients and in those with a low risk of dislocation.

LEVEL OF EVIDENCE

Level IV, therapeutic study. See Guidelines for Authors for a complete description of levels of evidence.

Biomechanical modeling of acetabular component polyethylene stresses, fracture risk, and wear rate following press-fit implantation

Press-fit implantation may result in acetabular component deformation between the ischial-ilial columns ("pinching"). The biomechanical and clinical consequences of liner pinching due to press-fit implantation have not been well studied. We compared the effects of pinching on the polyethylene fracture risk, potential wear rate, and stresses for two different thickness liners using computational methods. Line-to-line ("no pinch") reaming and 2 mm underreaming press fit ("pinch") conditions were examined for Trident cups with X3 polyethylene liner wall thicknesses of 5.9 mm (36E) and 3.8 mm (40E). Press-fit cup deformations were measured from a foam block configuration. A hybrid material model, calibrated to experimentally determined stress-strain behavior of sequentially annealed polyethylene, was applied to the computational model. Molecular chain stretch did not exceed the fracture threshold in any cases. Nominal shell pinch of 0.28 mm was estimated to increase the volumetric wear rate by 70% for both cups and peak contact stresses by 140 and 170% for the 5.9 and 3.8 mm-thick liners, respectively. Although pinching increases liner stresses, polyethylene fracture is highly unlikely, and the volumetric wear rates are likely to be low compared to conventional polyethylene.

Third-body abrasive wear challenge of 32 mm conventional and 44 mm highly crosslinked polyethylene liners in a hip simulator model

Hip simulator studies have shown that wear in the polyethylene liners used for total hip replacements increased with the larger-diameter femoral balls and could also be exacerbated by third-body abrasion. However, they also indicated that the more highly cross-linked polyethylene (HXPE) bearings were more wear resistant than conventional polyethylene (CXPE) bearings. Unfortunately the HXPE bearings appeared to be particularly sensitive to adverse wear conditions. One simulator study in particular indicated that poly(methyl methacrylate) (PMMA) debris increased wear sixfold by means of two-body abrasive interactions rather than the supposed third-body abrasion or roughening effects of the Co-Cr surfaces. There has been no confirmation of such novel theories. Therefore the goal of this study was to investigate the sensitivity of large-diameter HXPE bearings to the third-body PMMA wear challenge in a hip simulator model. An orbital hip simulator was used in standard test mode with a physiological load profile. The 32 mm control liners were machined from moulded GUR1050 and gamma irradiated to 35 kGy under nitrogen (CXPE). The 44 mm liners were also from moulded blanks, gamma irradiated to 75 kGy, machined to shape, given a proprietary heat treatment, and sterilized by gas plasma (HXPE). As in the published simulator model, the study was conducted in three phases. In phase 1, all cups were run in standard ('clean') lubricant for 1.5 x 10(6) cycles duration. In phase 2, three CXPE cups and six HXPE cups were run for 2 x 10(6) cycles with a slurry of PMMA particles added to the lubricant. In phase 3, the implants were again run in 'clean' lubricant for 2 x 10(6) cycles duration. In addition, three HXPE cups were run as wear controls for 5.5 x 10(6) cycles duration in clean lubricant. In phase-1, the HXPE liners demonstrated twelvefold reduced wear compared with the CXPE controls. The 32 mm and 44 mm Co-Cr balls were judged of comparable roughnesses. However, the surface finish of HXPE liners was superior to that of CXPE liners. In phase-2 abrasion, wear rates increased sixfold and eighty-fold for CXPE and HXPE bearings respectively. These data confirmed that HXPE bearings were particularly sensitive to 'severe' test modes. The Co-Cr balls revealed numerous surface patches representing transferred PMMA with average transient roughness increased to 25 nm and 212 nm for the 32 mm and 44 mm balls respectively. These PMMA patches produced an aggressive two-body abrasion wear of the polyethylene. After cleaning, the ball roughness returned to near normal. Therefore the Co-Cr roughness was not an issue in this severe test mode. In phase 3, the wear decreased to near the index values of phase 1, while liner roughness dropped by more than 90 per cent. The control CXPE liners now demonstrated twice the wear of the HXPE, as would be predicted comparing the diameter and cross-linking algorithms. No previous study has correlated polyethylene roughness profiles to wear performance. In phase 2, PMMA abrasion created significant damage to the polyethylene surfaces. The average roughness Sa of CXPE liners increased to 3.6 microm, a twenty-four-fold increase with some scratches up to 40 microm deep. The HXPE roughness also increased but only to 1.5 microm, a ninefold increase. The scratch indices Sz and Sp for HXPE surfaces were also 50 per cent less severe than on CXPE surfaces. However, within 2 x 10(6) cycles duration of phase 3, all liners had recovered to virtually their original surface finish in phase 1. In all test phases, the surface finish of the HXPE liners remained superior to control liners. These experimental data confirmed many of the results from the previous simulator study with the PMMA abrasion models. Thus the 44 mm liners appeared an excellent clinical alternative to the smaller ball designs used in total hip replacements.

The first 50 years of total hip arthroplasty: lessons learned

Fifty years have passed since the first total hip arthroplasty of the modern era was performed. At this, the vantage point, it is reasonable to review these five decades, inquiring behind the single dominating observation that, in its current form, this operation is one of the most successful of all surgical procedures for the management of end-stage human disease. What are the generic lessons that can be derived from the experience? Succinctly, five major observations appear valuable. They are "skunk works," "Pasteur's motto," "the totally unexpected," "research solutions," and "the role of alternatives." "Skunk works," an industrial management term, might be characterized as an innovative endeavor that is offline and off-budget resulting from the relentless pursuit of a vivid dream by creative zealots who eschew defeat. Pasteur's motto dealt with serendipity, which was crucial to total hip arthroplasty progress. The totally unexpected is represented by an entirely new manmade disease, "periprosthetic osteolysis." The research solutions are represented by the complex, sophisticated contemporary research that has unraveled periprosthetic osteolysis and suggested modes of correction. Finally, the application of "alternatives" has characterized major progress. Importantly, these, or similar generic observations, may provide insights into important progress in the future.

Anatomic diameter femoral heads in total hip arthroplasty: a preliminary report

BACKGROUND

The stability of total hip replacements has been directly related to the diameter of the femoral head in several studies; however, durability has necessitated the use of femoral heads with a relatively small diameter. Recent developments in metal-on-metal technology have allowed for the use of femoral head bearings that are anatomic in diameter. In this case series, we report on the early results of patients who were at greater risk for dislocation because of anatomic deficiencies or increased range-of-motion activities and underwent hip arthroplasty with implants that had articulating surfaces approaching anatomic dimensions.

METHODS

Thirty-four patients underwent forty total hip arthroplasties with use of a modular metal-on-metal articulation with an anatomic diameter femoral head and a press-fit stem. Thirty patients were active, and four patients were profoundly disabled and had bone or soft-tissue deficiencies that would increase the risk for dislocation. Dislocation precautions were maintained for six weeks, and patients were allowed extreme ranges of motion at three months.

RESULTS

There were no dislocations. Active patients continued in extreme range-of-motion activities. Disabled patients improved but were limited by their comorbidities.

CONCLUSIONS

Anatomic diameter femoral heads offer distinct theoretical advantages in total hip arthroplasty. These short-term results are encouraging, and further study of this new technology in a larger series with a longer follow-up period is warranted.

The use of a dual-articulation acetabular cup system to prevent dislocation after primary total hip arthroplasty: analysis of 384 cases at a mean follow-up of 15 years

The concept of a dual articulation acetabular cup was developed by Prof. Bousquet in 1974. This concept has been shown to provide high stability after revision and primary total hip arthroplasty. The aim of our study was to evaluate the incidence of prosthetic instability in a consecutive homogeneous series of 384 primary dual mobility cups. Incidence of instability and implant survival were evaluated. Mean follow-up was 15.3 years (range, 12-20). There was no early or late instability. On the acetabular side there were 13 aseptic loosenings, 14 intraprosthetic dislocations, and seven polyethylene wear cases that required replacement of the liner. The cumulative survival rate of the dual-articulation acetabular cup using surgical revision for aseptic loosening as the endpoint was 95.9% +/- 4.1% at 18 years postoperatively. Our series proves the good long term behaviour of dual-articulation acetabular components in primary arthroplasty. Their excellent survivorship rate and the absence of episodes of prosthetic instability increase our confidence in this concept.

Clinical performance of highly cross-linked polyethylenes in total hip arthroplasty

Aseptic loosening secondary to wear-debris-induced osteolysis has been identified as the leading cause of late failure of total hip arthroplasty. Highly cross-linked polyethylene acetabular liners were developed as one approach to reducing this wear. Preclinical laboratory wear testing showed a number of cross-linked polyethylenes to have dramatically less wear than the polyethylene that had been in use for several decades. After the initial bedding-in phase (one to two years), the percent reductions in the wear rate, as indicated by the amount of penetration of the head into the socket evident on serial radiographs, have been comparable with what was predicted from preclinical hip-simulator testing of the highly cross-linked polyethylenes. To our knowledge, there have been no reports of clinically relevant osteolysis that was clearly attributable to wear of a highly cross-linked polyethylene acetabular liner. However, the clinical performance of these materials should be closely monitored with long-term follow-up.

Analysis of retrieved acetabular components of three polyethylene types

The polyethylene used in total hip arthroplasty has gone through many changes over the past several decades, including consolidation processes, resin types, method of sterilization, packaging, and the extent of crosslinking. To isolate the in vivo performance of material changes from implant system design changes, we assessed the postretrieval surface wear and damage of components made from three different polyethylene types used in a single implant system. The polyethylene types investigated are representative of the sequentially available bearing materials that have dominated use in total hip arthroplasty over the last several decades. Forty-six components with implantation durations of 12 to 96 months were assessed for surface wear and damage and for socket wear and creep volume change. Acetabular components made from highly crosslinked polyethylene had a 50% lower total damage score than components made from polyethylene that was either gamma-sterilized in air or in nitrogen. The wear and creep socket volume change was 80% and 90% lower for the highly crosslinked components compared with the gamma-sterilized in air and nitrogen groups, respectively. These data of direct component measurement are consistent with earlier predictions that recent changes in polyethylene material processing can lead to clinically improved bearing performance.

Unconstrained tripolar implants for primary total hip arthroplasty in patients at risk for dislocation

We performed a retrospective study on 167 primary total hip arthroplasty (THA) procedures in 163 patients at high risk for instability to assess the reliability of unconstrained tripolar implants (press-fit outer metal shell articulating a bipolar polyethylene component) in preventing dislocations. Eighty-four percent of the patients had at least 2 risk factors for dislocation. The mean follow-up length was 40.2 months. No dislocation was observed. Harris hip scores improved significantly. Six hips were revised, and no aseptic loosening of the cup was observed. The tripolar implant was extremely successful in achieving stability. However, because of the current lack of data documenting polyethylene wear at additional bearing, the routine use of tripolar implants in primary THA is discouraged and should be considered at the present time only for selected patients at high risk for dislocation and with limited activities.

Radiostereometric analysis comparison of wear of highly cross-linked polyethylene against 36- vs 28-mm femoral heads

This study used radiostereometric analysis (RSA) to compare the femoral head penetration of 28- vs 36-mm-diameter femoral heads into highly cross-linked polyethylene in 2 groups of total hip arthroplasty patients. Thirty patients were enrolled in this RSA study using highly cross-linked polyethylene (Longevity, Zimmer Inc, Warsaw, Idaho) against either 28- or 36-mm-diameter cobalt chrome femoral heads. At 3-year follow-up, there was no significant difference in the total average femoral head penetration, including both creep and wear, using 3 methods of RSA measurement between the 2 groups. Importantly, after bedding-in, there was no further significant increase in the amount of femoral head penetration (ie, wear) with either head size between years 1 and 3. There were no radiographic signs of lysis or radiolucencies at a minimum 3-year follow-up.

Comparison of hip simulator wear of 2 different highly cross-linked ultra high molecular weight polyethylene acetabular components using both 32- and 38-mm femoral heads

Much evidence suggests that ultra high molecular weight polyethylene (UHMWP) irradiated to 9.5 Mrad has lower wear than UHMWP given 5 Mrad. Curiously, highly cross-linked polyethylenes gain weight during hip simulator testing. We postulated that (a) UHMWP irradiated to 9.5 Mrad would wear less than UHMWP irradiated to 5 Mrad UHMWP, (b) articulation against 38-mm heads would accentuate this difference, and (c) weight gain of highly cross-linked material reflects the inadequacy of load soak controls. We compared 9.5 versus 5 Mrad polyethylene in hip simulator wear tests, with both load soak corrections and with novel "motion soak" corrections. The 9.5-Mrad material wore less than 5-Mrad material for 32- and 38-mm heads. Motion soak corrections were more accurate than load soak corrections.

Influence of femoral head size on impingement, dislocation and stress distribution in total hip replacement

Dislocation remains a serious complication of total hip replacement. An insufficient range of motion can lead to impingement of the prosthetic neck on the acetabular cup. Together with the initiation of subluxation and dislocation, recurrent impingement can cause material failure in the liner. The objective of this study was to generate a validated finite element (FE) model capable of predicting the dislocation stability of different femoral head sizes with regard to impingement in different implant positions as well as the corresponding stress distribution in the liner. In order to cover posterior and anterior dislocation, two total hip dislocation associated manoeuvres were simulated using a three-dimensional nonlinear finite element model. The dislocation stability of two head sizes was determined numerically and experimentally. After validation, the FE model was used to analyse the dislocation stability of four different head sizes in variable implant positions. Range of motion (ROM) until impingement, the resisting moment that was developed and ROM until dislocation were evaluated. Additionally, stress distribution within the polyethylene liner during impingement and subluxation was determined. For both dislocation modes, a cup position of 45 degrees lateral abduction and 15 degrees up to 30 degrees anteversion resulted in appropriate ROM and dislocation stability. In general, larger head diameters revealed an increase in ROM and higher resisting moments. Stress analysis showed decreased contact pressures at the egress site of the liners with the larger inner diameters during subluxation. The analysis shows that an optimal implant position and a larger head diameter can reduce the risk of dislocation induced by impingement. The finite element model that was developed enables simplification of design variations compared to experimental studies since prototyping and assembling are replaced by prompt numerical simulation.

Dislocation of primary total hip arthroplasty with 36 and 40-mm femoral heads

Some authors suggest the use of larger (36 and 40 mm) femoral heads against highly cross-linked polyethylene acetabular liners will reduce the prevalence of early dislocation after primary total hip arthroplasty (THA). We prospectively followed 61 consecutive patients at high risk for dislocation to determine the prevalence of early dislocation after primary THA with 36 and 40 mm femoral heads. We established specific indications for the use of these larger heads in primary arthroplasty. There were 65 hips in 61 patients with a minimum followup of 1 year (mean 2 years, range 1-4 years). There were 55 36 mm heads and 10 40 mm heads. All arthroplasties were performed by the posterior approach with capsular repair. The prevalence of early dislocation was 4.6% (3 of 65). There was one patient with an anterior dislocation and two with posterior dislocation. No patient underwent reoperation for dislocation. There was no dislocation in the 10 hips that had a 40 mm head. We identified no complications related to the larger femoral heads. The use of larger femoral heads did not notably reduce the prevalence of early dislocation after primary THA in high risk patients compared to historical controls.

In vitro comparison of frictional torque and torsional resistance of aged conventional gamma-in-nitrogen sterilized polyethylene versus aged highly crosslinked polyethylene articulating against head sizes larger than 32 mm

BACKGROUND

The advent of highly crosslinked polyethylene has allowed the re-evaluation of the use of femoral heads larger than 32 mm for metal-on-polyethylene total hip arthroplasties. However, the effect of larger heads on the frictional torque of highly crosslinked polyethylene is unknown.

METHODS

We performed an in vitro examination of the effect of larger chrome cobalt femoral heads (40 mm diameter) on the frictional torque and torsional resistance of hip articulations on aged liners of polyethylene that were sterilized by gamma rays while in nitrogen, and aged highly crosslinked polyethylene. The frictional torque at the femoral head articulation was usually higher for the highly crosslinked polyethylene than for the conventional polyethylene. The aged conventional liners oxidized considerably, which led to gross failure of the polyethylene at the anti-rotation portion of the rim. The aged crosslinked polyethylene showed no such failures despite the higher frictional torque.

INTERPRETATION

Our findings suggest that in terms of torsional resistance to fatigue when studied as a device, rather than as an isolated material, under these conditions, aged highly crosslinked polyethylene is preferable to aged conventional polyethylene.

Contact stress assessment of conventional and highly crosslinked ultra high molecular weight polyethylene acetabular liners with finite element analysis and pressure sensitive film

Stress magnitude and distribution of both conventional polyethylene versus a crosslinked polyethylene in the liner of a total hip replacement (THR) were examined using finite element analysis and pressure sensitive film. Both types of polyethylene were assessed against head sizes of 22 and 28 mm with 5-mm thick polyethylene liners and head sizes of 28, 38, and 46 mm with 3-mm thick polyethylene liners. Liners with 5-mm conventional polyethylene represented successful combinations with long track records. Our hypothesis was that although the combination of the large head and the lower modulus of the highly crosslinked polyethylene would lead to lower stresses, the stresses would be excessive if the liner was extremely thin at 3 mm. Von Mises stresses at the articulating surface of the highly crosslinked liners were lower, when compared to conventional polyethylene, in every THR size examined. Specifically, however, the 38- and 46-mm inner diameter (ID) highly crosslinked polyethylene even at the extreme of only 3-mm thick had lower stresses than the 22-mm ID conventional liner of 5-mm thickness. These data indicate that the use of a large head against highly crosslinked material even at 3-mm thickness results in lower stresses than in an existing conventional 22-mm head and 5-mm thick combination. Obviously, other considerations will influence the minimum thickness to be recommended.

Third-body wear testing of a highly cross-linked acetabular liner: the effect of large femoral head size in the presence of particulate poly(methyl-methacrylate) debris

The hip simulator wear performance of an electron beam cross-linked and subsequently melted ultrahigh molecular weight polyethylene against femoral heads of 28-, 38-, and 46-mm diameter in the presence of poly(methyl-methacrylate) particulate debris was contrasted with that of conventional polyethylene against a 46-mm diameter head. Over 5 million cycles of testing, the average wear rate of the conventional polyethylene liners was 29.3 +/- 3.0 mg per million cycles. All highly cross-linked components exhibited marked reduction in wear, with the highest wear measuring 0.74 +/- 0.85 mg per million cycles. This study, using a clinically relevant third-body material, showed the electron beam cross-linked material to be far more resistant to this third-body wear than conventional ultrahigh molecular weight polyethylene, even when very large diameter femoral heads were used.

A review of current cross-linked polyethylenes used in total joint arthroplasty

Major improvements have been made in new polyethylenes in regards to wear resistance and oxidation resistance. The background for and explanation of these improvements are presented in this study. The various manufacturing processes are described along with the possible features that the differences in manufacturing processes may have on wear, rate of particle generation, oxidation and mechanical properties. The role of some of the new polyethylenes in permitting the use of larger-diameter heads and the advantages of large-head diameters are discussed. Some of the advantages of metal-on-polyethylene versus hard-on-hard bearings also are described.

Highly cross-linked, electron-beam-irradiated, melted polyethylene: some pros

Extensive new evidence generated within the past the year provides strong support for the use of electron-beam highly cross-linked, subsequently melted ultra-high molecular weight polyethylene in total hip replacement arthroplasty. In terms of wear reduction, three studies involving three different demographic groups and two different measurement techniques have found that the femoral head penetration with this type of polyethylene after bedding in has finished taking place is less than 10 micra per year. This wear rate is similar to the wear rate of metal-on-metal and ceramic-on-ceramic articulations. Retrieval specimens up to 3 years after insertion confirmed the minimal wear by exhibiting persisting machine marks throughout the inside diameter of the liner. Extensive studies show no evidence of oxidation, confirming the absence of residual free radicals. No evidence of fatigue failure exists except three known cases out of 150,000, in which malposition of the acetabular component produced abnormally high contact stresses on unsupported polyethylene. The in vivo wear, oxidation resistance, and mechanical properties of this alternative bearing material are excellent, with in vivo durations now exceeding five years. The other major advantages over hard-on-hard bearings including familiarity, adaptability, forgiveness, and cost seem to be compelling.

Early morbidity of modular exchange for polyethylene wear and osteolysis

This study assessed the early morbidity associated with modular component exchange surgery for the treatment of accelerated polyethylene wear and osteolysis in 55 patients. Review of the surgical records revealed no significant intraoperative complications, little intraoperative blood loss (mean 333 mL), no allogenic blood transfusions, and no recorded postoperative deep vein thromboses. Eighteen percent of patients, however, experienced postoperative dislocation. Five patients dislocated multiple times, 3 of which required rerevision surgery. Two patients required rerevision for femoral implant fractures related to osteolysis and 1 additional patient required rerevision due to catastrophic failure of the acetabular component 5 years postoperatively. With an average follow-up of 30 months, 6 of the 55 patients treated with modular exchange required rerevision. The results of this study suggest that instability is the most prevalent early complication associated with modular component exchange. As such, we believe that more stable constructs should be emphasized, possibly at the expense of polyethylene thickness.

Dislocation after total hip arthroplasty

Dislocation is one of the most common complications after total hip arthroplasty (THA). Risk factors include neuromuscular and cognitive disorders, patient non-compliance, and previous hip surgery. Surgical considerations that must be addressed include approach, soft-tissue tension, component positioning, impingement, head size, acetabular liner profile, and surgeon experience. Recent improvements in posterior soft-tissue repair after primary THA have shown a reduced incidence of dislocation. When dislocation occurs, a thorough history, physical examination, and radiographic assessment help in choosing the proper intervention. Closed reduction usually is possible, and nonsurgical management frequently succeeds in preventing recurrence. When these measures fail, first-line revision options should target the underlying etiology. This most often involves tensioning or augmentation of soft tissues, as in capsulorrhaphy or trochanteric advancement; correction of malpositioned components; or improving the head-to-neck ratio. If instability persists, or if a primary THA repeatedly dislocates without a clear cause, a constrained cup or bipolar femoral prosthesis may be as effective as a salvage procedure.