Risk Factors Affecting Outcome of Metal-on-Metal Surface Arthroplasty of the Hip:

Comparison of acetabular reamings during hip resurfacing versus uncemented total hip arthroplasty

PURPOSE

To compare the quantity of bone removed from the acetabulum during resurfacing hip arthroplasty versus uncemented total hip arthroplasty (THA).

METHODS

62 consecutive patients with osteoarthritis of the hip were prospectively studied. 24 men and 7 women aged 40 to 86 (mean, 59) years underwent Birmingham hip resurfacing. 13 men and 18 women aged 34 to 88 (mean, 61) years underwent uncemented THA using the trident acetabular cup. Obese elderly women at risk of femoral neck fracture and patients with large subchondral pseudocysts or a history of avascular necrosis of the femoral head were assigned to uncemented THA. Acetabular reamings were collected; marginal osteophytes were not included. The reamings were dehydrated, defatted, and weighed.

RESULTS

The mean weight of acetabular reamings was not significantly different between patients undergoing hip resurfacing and uncemented THA (p=0.57).

CONCLUSION

In hip resurfacing, the use of an appropriately small femoral component avoids oversizing the acetabular component and removal of excessive bone stock.

Hip resurfacing arthroplasty: risk factors for failure over 25 years

Many early metal-on-polyethylene hip resurfacing arthroplasty designs were abandoned after reports of high short-term and midterm failure rates. To investigate factors associated with failure, we retrospectively reviewed our experience with early-design hip resurfacing implants in 75 patients during a 25-year period (median followup, 7.9 years; range, 0.1-25.2 years). Implant failure was defined as revision for any reason. One of 75 patients was lost to followup. The estimated rate of implant survival was 73% at 5 years, 34% at 10 years, 27% at 15 years, 12% at 20 years, and 8% at 25 years. Of the many clinical and radiographic factors considered, only age, implant type, and gender were associated with implant survival independent of other variables considered. Hip resurfacing arthroplasty showed poor overall long-term survival in this series. Particular attention should be paid to the identified risk factors as long-term followup data become available for modern designs.

LEVEL OF EVIDENCE

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

Component alignment in hip resurfacing using computer navigation

The use of computer navigation during hip resurfacing has been proposed to reduce the risk of a malaligned component and notching with subsequent postoperative femoral neck fracture. Femoral component malalignment and notching have been identified as the major factors associated with femoral neck fracture after hip resurfacing. We performed 37 hip resurfacing procedures using an imageless computer navigation system. Preoperatively, we generated a patient-specific computer model of the proximal femur and planned a target angle for placement of the femoral component in the coronal plane. The mean navigation angle after implantation (135.5 degrees) correlated with the target stem-shaft angle (135.4 degrees). After implantation, the mean stem-shaft angle of the femoral component measured by three-dimensional computed tomography (135.1 degrees) correlated with the navigation target stem-shaft angle (135.4 degrees). The computer navigation system generates a reliable model of the proximal femur. It allows accurate placement of the femoral component and provides precise measurement of implant alignment during hip resurfacing, thereby reducing the risk of component malpositioning and femoral neck notching.

The reliability of radiographic assessment of femoral neck-shaft and implant angulation in hip resurfacing arthroplasty

Fifteen sets of patient radiographs were analyzed by 3 different observers on 2 occasions. Each observer measured the femoral neck-shaft angles (NSAs) of the preoperative digital radiographs and stem-shaft angles (SSAs) of the postoperative radiographs. The effect of femur position on SSA measured by digital radiographs was also investigated using a resurfaced synthetic femur. Radiographs were taken with the synthetic specimen positioned in 10 degrees increments of either flexion or rotation. Measurement by digital radiographs proved less than optimal in assessing preoperative NSA but was better in assessing the postoperative component SSA. External rotation of 30 degrees and flexion of 40 degrees resulted in a clinically significant disparity in SSA measurements. Patient malposition during radiographic imaging can contribute to erroneous NSA and SSA results.

Imageless computer navigation for placement of the femoral component in resurfacing arthroplasty of the hip

We have investigated the accuracy of placement of the femoral component using imageless navigation in 100 consecutive Birmingham Hip Resurfacings. Pre-operative templating determined the native neck-shaft angle and planned stem-shaft angle of the implant. The latter were verified post-operatively using digital anteroposterior unilateral radiographs of the hip. The mean neck-shaft angle determined before operation was 132.7 degrees (118 degrees to 160 degrees ). The mean planned stem-shaft angle was a relative valgus alignment of 9.7 degrees (SD 2.6). The stem-shaft angle after operation differed from that planned by a mean of 2.8 degrees (SD 2.0) and in 86% of cases the final angle measured within +/- 5 degrees of that planned. We had no instances of notching of the neck or varus alignment of the implant in our series. A learning curve was observed in the time taken for navigation, but not for accurate placement of the implant. Navigation in hip resurfacing may afford the surgeon a reliable and accurate method of placement of the femoral component.

A Biomechanical and Finite Element Analysis of Femoral Neck Notching During Hip Resurfacing

Hip resurfacing is an alternative to total hip arthroplasty in which the femoral head surface is replaced with a metallic shell, thus preserving most of the proximal femoral bone stock. Accidental notching of the femoral neck during the procedure may predispose it to fracture. We examined the effect of neck notching on the strength of the proximal femur. Six composite femurs were prepared without a superior femoral neck notch, six were prepared in an inferiorly translated position to create a 2 mm notch, and six were prepared with a 5 mm notch. Six intact synthetic femurs were also tested. The samples were loaded to failure axially. A finite element model of a composite femur with increasing superior notch depths computed maximum equivalent stress and strain distributions. Experimental results showed that resurfaced synthetic femurs were significantly weaker than intact femurs (mean failure of 7034 N, p<0.001). The 2 mm notched group (mean failure of 4034 N) was significantly weaker than the un-notched group (mean failure of 5302 N, p=0.018). The 5 mm notched group (mean failure of 2808 N) was also significantly weaker than both the un-notched and the 2 mm notched groups (p<0.001, p=0.023, respectively). The finite element model showed the maximum equivalent strain in the superior reamed cancellous bone increasing with corresponding notch size. Fracture patterns inferred from equivalent stress distributions were consistent with those obtained from mechanical testing. A superior notch of 2 mm weakened the proximal femur by 24%, and a 5 mm notch weakened it by 47%. The finite element analysis substantiates this showing increasing stress and strain distributions within the prepared femoral neck with increasing notch depth.

Resurfacing is comparable to total hip arthroplasty at short-term follow-up

Metal-on-metal total hip resurfacing arthroplasty has had excellent reported results at early to midterm followup, and some studies suggest that outcomes are comparable to conventional THA. We compared the clinical and radiographic outcomes of two closely matched groups of 54 patients who underwent resurfacing and conventional THA, respectively. Each group consisted of 36 men and 18 women who had a mean age of 52 years and a mean body mass index of 29 kg/m(2). At a minimum followup of 24 months (mean, 40 months; range, 24-60 months), the mean Harris hip scores increased similarly in both groups (from 52 to 90 points and from 50 to 91 points for the resurfacing and conventional groups, respectively). Radiographic outcomes, revision rates, complications, pain scores, and satisfaction ratings of the two groups were similar. The patients who underwent resurfacing had higher postoperative weighted activity scores than the patients who underwent conventional THA, although they had higher preoperative weighted activity scores as well. The early outcomes of resurfacing are comparable to those of conventional THA.

LEVEL OF EVIDENCE

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

2008 Otto Aufranc Award: component design and technique affect cement penetration in hip resurfacing

Either excessive or insufficient cement penetration within the femoral head after hip resurfacing influences the risk of femoral failures. However, the factors controlling cement penetration are not yet fully understood. We determined the effect of femoral component design and cementation technique on cement penetration. Six retrieved femoral heads were resurfaced for each implant (BHR, ASR, Conserve Plus, DuROM, ReCAP) using the manufacturers' recommendations for implantation. In addition, the BHR was implanted using the Conserve Plus high-viscosity cementation technique, "BHR/hvt," and vice versa for the Conserve, "Conserve/lvt." The average cement penetration was highest with BHR (65.62% +/- 15.16%) compared with ASR (12.25% +/- 5.12%), Conserve Plus(R) (19.43% +/- 5.28%), DuROM (17.73% +/- 3.96%), and ReCAP (26.09% +/- 5.20%). Cement penetration in BHR/hvt remained higher than all other implants equaling 36.7% +/- 6.6%. Greater femoral component design clearance correlated with cement mantle thickness. Femoral component design in hip resurfacing plays a major role in cement penetration.

Is patient selection important for hip resurfacing?

The optimal implant option for hip arthroplasty in the young, active patient remains controversial. There has been renewed interest for metal-on-metal hip resurfacing due to improved design and manufacturing of implants, better materials, enhanced implant fixation, theoretical advantages over conventional total hip arthroplasty, and recent Food and Drug Administration approval of two devices. Recent studies indicate satisfactory short- and midterm clinical results (1- to 10-year followup) with low complication rates, but there is a learning curve associated with this procedure, a more extensive surgical approach is necessary, and long-term results have yet to be determined. Proper patient selection may help avoid complications and improve patient outcomes. Patient selection criteria in the literature appear based predominantly on theoretical considerations without any consensus on stratifying patient risk. The most commonly reported complications encountered with hip resurfacing include femoral neck fracture, acetabular component loosening, metal hypersensitivity, dislocation, and nerve injury. At the time of clinical evaluation, patient age; gender; diagnosis; bone density, quality, and morphology; activity level; leg lengths; renal function; and metal hypersensitivity are important factors when considering a patient for hip resurfacing. Based on our review, we believe the best candidates for hip resurfacing are men under age 65 with osteoarthritis and relatively normal bony morphology.

LEVEL OF EVIDENCE

Level V, prognostic study. See the Guidelines for Authors for a complete description of levels of evidence.

Metal-on-metal hip resurfacing arthroplasty: a review of periprosthetic biological reactions

Metal-on-metal hip resurfacing arthroplasty has undergone a recent resurgence as an alternative treatment option for young and active patients with significant osteoarthritis. The claimed advantages of metal-on-metal hip resurfacing arthroplasty include lower wear rate, preservation of bone stock for subsequent revision procedures, restoration of anatomic hip mechanics, and enhanced stability due to the larger diameter of articulation. A disadvantage, however, is that the metal-on-metal resurfacing releases large amounts of very small wear particles and metal ions. The long-term biological consequences of the exposure to these Co-Cr particles and ions remain largely unknown. The purpose of this review is to provide an overview of the current literature on the adverse periprosthetic biological reactions associated with metal-on-metal hip resurfacing arthroplasty.

Metal-on-metal hip resurfacing radiographic evaluation techniques

Hip resurfacing devices require a new radiographic evaluation technique owing to femoral components with short or no stems. Fourteen US surgeons implanted 1148 metal-on-metal hip resurfacing (HR) devices in a US-FDA-IDE clinical trial, which began in 2001. In this multi-center, prospective study, 337 patients (mean age, 50.1 years) were enrolled as a study group of unilateral HR arthroplasties. Radiographs of 292 HR arthroplasties at a minimum 2-year follow-up (maximum 3 years) were reviewed. There were 10 patients with radiographic evidence of femoral component instability beyond 2 years, as evidenced by subsidence > or = 5mm. Of these, 7 did not have clinical symptoms associated with femoral component instability. In the study group, 24 revisions were reported, of which 8 were due to femoral neck fractures, 4 were due to acetabular component loosening, 11 were due to femoral component loosening, and 1 due to dislocation.

Femoral neck fracture following hip resurfacing

A total of 20 pairs of fresh-frozen cadaver femurs were assigned to four alignment groups consisting of relative varus (10° and 20°) and relative valgus (10° and 20°), 75 composite femurs of two neck geometries were also used. In both the cadaver and the composite femurs, placing the component in 20° of valgus resulted in a significant increase in load to failure. Placing the component in 10° of valgus had no appreciable effect on increasing the load to failure except in the composite femurs with varus native femoral necks. Specimens in 10° of varus were significantly weaker than the neutrally-aligned specimens.

The results suggest that retention of the intact proximal femoral strength occurs at an implant angulation of ≥ 142°. However, the benefit of extreme valgus alignment may be outweighed in clinical practice by the risk of superior femoral neck notching, which was avoided in this study.

[Blood flow to the femoral head and hip resurfacing arthroplasty]

After disastrous outcomes due to wear-induced osteolysis in the 1980s, hip resurfacing arthroplasty has undergone a renaissance, mainly because of the introduction of metal-on-metal bearings. However, there are still problems associated with this technique, such as femoral neck fractures, neck thinning, and aseptic loosening, and their causes are still being investigated. During the last years, increasing evidence has shown that both the frequently used posterior approach to the hip as well as preparation of the femoral epiphysis can impair blood supply to the femoral head. In the presence of mechanical stresses (impaction, heat development during cement polymerization) during the implantation, the epiphysis might not be able to compensate for this. This paper summarizes the current understanding of the blood supply to the femoral head and proposes a way to preserve the viability after hip resurfacing.

Metal-on-metal hip resurfacing: advantages and disadvantages

Modern metal-on-metal resurfacing has recently gained popularity as an alternative to standard stemmed total hip arthroplasty. This study analyzed, from a literature review, the purported advantages and disadvantages of resurfacing with a comparison to standard hip arthroplasty. Advantages may include bone conservation on the femoral side with possible lower dislocation rates, more range-of-motion, more normal gait pattern, increased activity levels, increased ease of insertion with proximal femoral deformities or retained hardware, and straightforward revision. Possible disadvantages of resurfacing are increased difficulty to perform the procedure, increased acetabular bone stock loss, femoral neck fractures, and concerns about the effects of metal ions. Many of these issues will need further clarification by well-planned prospective studies and evaluation of longer-term outcomes.

The influence of age and sex on early clinical results after hip resurfacing: an independent center analysis

Patient selection is critical to the excellent medium-term clinical results after hip resurfacing. We assessed the influence of age and sex on early survivorship and functional outcome by comparing 100 female hips resurfaced with male hips resurfaced for the same period. In patients older than 55 years, Harris hip score improved to 97.4 in males compared with 91.2 (P < .01) in females with a revision rate of 2.2% and 7.4%, respectively. There was no correlation between age and functional score. Three percent of females and 1.3% of males sustained a femoral neck fracture. Hip resurfacing provides excellent early functional recovery in males and females. However, the revision rate in older females is high. Changes to surgical technique may minimize the risk of early failure in this group.

Causes of early failure in a multicenter clinical trial of hip resurfacing

The purpose of this study was to evaluate the clinical outcomes and possible causes of early failure in a multicenter trial of metal-on-metal hip resurfacing. Two hundred patients were prospectively enrolled and followed for an average of 31.2 months (range, 12-54 months). Of 200 patients, 14 (7.0%) required revision surgery at a mean time of 19.5 months (range, 3-47 months). Patients with failures were significantly younger and heavier than the nonfailures, and all were male. Patients who were revised did not differ from those who were not revised in terms of radiographic outcomes, but they did report lower functional outcome scores at all preoperative and postoperative testing intervals. Most failures (10/14) were related to early acetabular loosening. The learning curve was likely a factor in these cup failures. This report highlights the importance of patient selection and surgical technique in hip resurfacing arthroplasty. It is anticipated that further surgical experience will lead to a reduction in this high early failure rate.

Birmingham hip resurfacing arthroplasty

We report the outcome at a minimum of five years of 110 consecutive metal-on-metal Birmingham Hip Resurfacing arthroplasties in 98 patients. The procedures were performed between October 1999 and June 2002 by one surgeon. All patients were followed up clinically and radiologically. The mean follow-up was 71 months (60 to 93). Revision of either component was defined as failure.

The mean Harris Hip score at follow-up was 96.4 (53 to 100). The mean Oxford hip score was 41.9 (16 to 57) pre-operatively and 15.4 (12 to 49) post-operatively (p < 0.001). The mean University of California Los Angeles activity score was 3.91 (1 to 10) pre-operatively and 7.5 (4 to 10) post-operatively (p < 0.001).

There were four failures giving a survival at five years of 96.3% (95% confidence interval 92.8 to 99.8). When applying a new method to estimate narrowing of the femoral neck we identified a 10% thinning of the femoral neck in 16 hips (14.5%), but the relevance of this finding to the long-term outcome remains unclear.

These good medium-term results from an independent centre confirm the original data from Birmingham.

Risk of periprosthetic femoral neck fracture after hip resurfacing arthroplasty: valgus compared with anatomic alignment. A biomechanical and clinical analysis

BACKGROUND

Early clinical results of hip resurfacing arthroplasty have led to the recommendation to achieve a neck-shaft angle of 140 degrees when inserting the femoral component. In addition, the idea of adhering to an absolute angle when inserting instrumentation in hips with excessive anatomic varus or valgus neck-shaft angles has raised concern. A biomechanical analysis was completed in order to determine if the achieved valgus orientation of the femoral component reduced the risk of periprosthetic fracture.

METHODS

Twenty fresh-frozen cadaveric femora were blindly assigned to be implanted with a neutral or valgus-oriented hip-resurfacing femoral component. Bone mineral density scans were acquired for all femora. All specimens were loaded axially to failure at a rate of 0.21 mm per second. Radiographs of the specimens were measured in order to determine the relative valgus orientation of the femoral components and the change in offset.

RESULTS

There was a significant increase in the ultimate failure load for the valgus-oriented components. While the bone density scans revealed that the bone mineral densities measured in the neutral and valgus-oriented femoral components were almost identical, the ultimate failure load was found to be significantly increased for the valgus-oriented components (6955 N) compared with the neutral-oriented components (5254 N). For the valgus-oriented femoral components, two had failure at the subcapital level, seven had vertical shear fractures, and one had an anterior shear fracture. For the neutral-oriented components, five subcapital fractures and five vertical shear failures were observed.

CONCLUSIONS

The study suggests that a valgus orientation decreases the risk of periprosthetic femoral neck fracture following hip resurfacing. It also brings into question the use of an absolute angle for all patients. Obtaining the maximum possible valgus angle, while avoiding notching, may in fact provide the optimum protection from periprosthetic femoral neck fractures.

Learning how to resurface cam-type femoral heads with acceptable accuracy and precision: the role of computed tomography-based navigation

BACKGROUND

Resurfacing arthroplasty for cam-type deformities, which are a common cause of early osteoarthritis, is a technically demanding operation. Like any other arthroplasty, it requires both accuracy and precision. On the basis of the results of series reported by expert surgeons, we considered it desirable that this operation should be performed within +/-10 degrees of the desired angular orientation and +/-6 mm of entry-point translation in 95% of hips. Technological aids are now available to help surgeons achieve that level of accuracy. Three models of cam-type hips of increasing severity were used to assess the efficacy of three systems of instrumentation at delivering the required level of accuracy and precision.

METHODS

Thirty-two students of surgical technology were instructed in hip resurfacing and shown detailed plans of the desired operative outcome for the three hips with cam-type deformity. They then used conventional instruments, imageless navigation, and computed tomography-based navigation to perform the operation as accurately as possible.

RESULTS

Conventional instrumentation produced an unacceptably wide range of entry-point errors. Imageless navigation was able to deliver adequate accuracy and precision in varus-valgus angulation and superoinferior translation, but was less satisfactory in version and anteroposterior translation. Computed tomography-based navigation enabled novice surgeons to navigate hips that had difficult cam-type deformity with acceptable precision in all four degrees of freedom measured.

CONCLUSIONS

Only computed tomography-based navigation appears to be appropriate for delivering both the accuracy and the precision needed by surgeons on the steep part of their learning curve. Neither conventional neck-based instrumentation nor imageless navigation provided enough help for novice surgeons learning to perform this technically challenging operation.

Improved survivorship of hybrid metal-on-metal hip resurfacing with second-generation techniques for Crowe-I and II developmental dysplasia of the hip

BACKGROUND

The purpose of this study was to determine the influence of improved femoral fixation techniques on the survivorship of metal-on-metal total hip resurfacing prostheses in patients with developmental dysplasia of the hip and to report the long-term results of our patients managed earlier with first-generation fixation techniques.

METHODS

One hundred and three hips (ninety patients) were resurfaced for osteoarthritis secondary to developmental dysplasia. The mean age of the patients was forty-seven years, and 77% were women. Most hips (94%) were Crowe class I, but 43% had femoral head defects of >1 cm in size. The clinical results of these hips were compared with those of a group of patients with other etiologies, largely dominated by idiopathic osteoarthritis (78%).

RESULTS

All clinical scores improved significantly (p < 0.0001) and were comparable with those of patients with other etiologies except for the postoperative activity scores, which were lower (7.0 compared with 7.5). Range of motion was greater for the patients with dysplasia than for the patients with other etiologies. Seven hips that were resurfaced with the first-generation femoral fixation techniques and one hip that was resurfaced with the second and third-generation techniques had conversion to total hip arthroplasty. This difference was found to be significant (p = 0.032) in a multivariate, time-dependent analysis after adjustment for other covariates known to affect prosthetic survival. There was no loosening of the acetabular component in this series.

CONCLUSIONS

The current improvements in the short-term to midterm results after resurfacing in patients with developmental dysplasia of the hip in whom more current techniques were used are encouraging and allow for greater expectations regarding the elimination of short-term failures and improved long-term durability of resurfacing in this population.

Does computer-assisted surgery improve accuracy and decrease the learning curve in hip resurfacing? A radiographic analysis

BACKGROUND

Hip resurfacing is a technically demanding procedure in which accurate positioning of the femoral component is critical to the avoidance of early implant failures. The purpose of this study was to assess the accuracy of computer-assisted placement of the femoral component and to evaluate the impact of computer-assisted surgery on the learning curve associated with this procedure.

METHODS

The accuracy of positioning the femoral component was analyzed radiographically in hips undergoing resurfacing procedures performed by surgeons assigned to four different study groups: Group 1, in which the operations were performed with use of computer-assisted surgery by a fellowship-trained surgeon who was experienced in performing resurfacing arthroplasty (surgical experience, more than 250 hip resurfacings); Group 2, in which the operations were performed with use of computer-assisted surgery by senior residents who were inexperienced in performing resurfacing arthroplasty and who were closely supervised by faculty; Group 3, in which the operations were performed with use of conventional instruments by fellowship-trained faculty members; and Group 4, in which the operations were performed with use of computer-assisted surgery by a lesser experienced fellowship-trained faculty member (surgical experience, more than forty but less than seventy-five hip resurfacings) from Group 3.

RESULTS

The range of error in varus or valgus angulation that was observed for navigated procedures was 6 degrees in Group 1, 7 degrees in Group 2, and 5 degrees in Group 4. Compared with the preoperative neck-shaft angle value, the mean postoperative stem-shaft angle value increased by a mean of 4.7 degrees in Group 1, 7.2 degrees in Group 2, 6.5 degrees in Group 3, and 11.6 degrees in Group 4. When compared with the use of standard instrumentation, the use of computer-assisted surgery reduced the number of outliers and facilitated valgus insertion.

CONCLUSIONS

In the present study, computer-assisted surgery resulted in improved accuracy and precision in positioning the femoral component. In addition, computer-assisted surgery led to a reduction in the length of the learning curve for beginners in hip resurfacing and improved the surgeon's ability to perform this procedure safely.

Have contemporary hip resurfacing designs reached maturity? A review

The increasing prevalence of hip replacements in young patients is expected to fuel the demand for hip resurfacing arthroplasty. Patient selection, surgical technique, and implant design can influence the clinical outcomes for these patients. In this review, we discuss whether contemporary hip resurfacing designs have reached maturity and suggest design considerations for future-generation implants. These design-related factors include the amount and extent of cement fixation, adoption of cementless femoral fixation, optimization of implant position, minimization of stress-shielding, improvement in modularity or sizing and geometry options, metallurgy, development of alternative bearing options, and examination of in vivo cup deformation. In addition, this review is based on an understanding of the causes of failure of revision hip resurfacing arthroplasty, which is essential to help guide research, implant design, and clinical decision-making.

[Fluoroscopic navigation versus conventional manual positioning of the femoral component for hip resurfacing: first experimental trial]

BACKGROUND

The most essential improvement of modern hip resurfacing arthroplasty is the metal-on-metal bearing as well as the integration of a procedure for the exact and repeatable positioning of the femoral component through a specific mechanical alignment instrument. Nevertheless, the main reasons for early implant failure are mal-positioning of the femoral component and notching of the femoral neck during femoral head preparation.

MATERIALS AND METHODS

In the context of an in vitro study, in each case six DUROM-Hip resurfacing prostheses were implanted in artificial femora with the prosthesis-specific mechanical alignment instrument, as well as under navigation control. The aim of the study was to evaluate the functionality and accuracy of a computer-assisted planning and navigation system on the basis of a navigation module library from Surgitaix AG (Aachen, Germany), as well as a comparison with the prosthesis-specific mechanical alignment instrument.

RESULTS

The main angulation error between planning and navigation of the stem-shaft angle was 0.2+/-1.2 degrees for the navigation system and 6.5+/-4.1 degrees for the mechanical alignment instrument, the main anterior offset error was 1.2+/-1.2 mm vs. -0.83+/-4.1 mm. The mean time for all five planning and navigation steps was 17+/-1.2 min vs. 14+/-0.8 min. The main distance error between planning and navigation was 1.9+/-0.6 mm for the navigation system, and 5.3+/-2.4 mm for the mechanical alignment instrument. Femoral notching was not observed for navigational or conventional positioning.

CONCLUSION

The computer-assisted fluoroscopic planning and navigation system for hip resurfacing showed, within the scope of this in vitro study, first promising experiences. The system approves a practicable planning with a high accuracy in implementation. Nevertheless, the potential benefit has to be evaluated in further clinical studies, especially from the perspective of a possible integration of this navigation system into the clinical workflow. Further studies should consider a fluoroscopy-assisted range of motion assessment under consideration of an additional cup-module to enhance the postoperative range of motion after hip resurfacing procedures.

[Metal-on-metal hybrid hip resurfacing. Development and current state]

Worldwide the employment of surface replacements using metal-on-metal components as an option, particularly for the young and active patient, has gained broad acceptance. Part of the attraction for hip resurfacing is its conservative nature as a prosthetic solution for hip arthritis. It is anatomical, replicating the normal hip and limb length, preserving proximal femoral bone, and is a truly minimally bone invasive approach with excellent outcome of joint function. The purpose of this article is to show the data of 1,000 Conserve(c) Plus hybrid metal-on-metal prostheses in a consecutive study of 1,140 patients with a follow-up of 5.6 years. The current Kaplan and Meier survival estimates of the prosthesis, using any conversion to total hip replacement as the end point, were 98.1% at 3 years [95% confidence interval (CI): 96.8-98.9%], 96.7% at 4 years (95% CI: 94.8-97.8%), and 95.2% at 5 years (95% CI: 93.0-96.8%). The mean postoperative Harris hip score was 93.3. The current state of metal-on-metal surface replacement is positive. The new generation of hip resurfacing has a lot of improvements. The purpose of this review of the procedure is to point out the definite improvements from earlier designs using polyethylene as well as to highlight the overall results and durability achieved by one surgeon's extensive experience and to assess the results from other series and centers. We also want to point out the areas where further investigation is needed.

[Hip resurfacing: current state of knowledge]

SUMMARY

The tribologic quality of metal-on-metal bearings has enabled a second generation of hip resurfacing techniques. Compared with a conventional hip prosthesis, this type of arthroplasty has many advantages: sparing femoral (and acetabular) bone stock, preservation of hip joint biomechanics (femoral offset, leg length), better recovery for high-level sports activities, easier revision, less risk of dislocation, less risk of extension to the shaft in the event of osteolysis. Hip resurfacing can thus be considered as true "minimally invasive bone surgery". There are however specific complications of resurfacing, including femoral neck fracture and collapse of the femoral head. All of the conventional approaches can be used for hip resurfacing procedures, but a precise operative technique is mandatory. The key to success is a proper position of the femoral piece. Certain biomechanical rules for implantation are required in order to limit the cam effect, spare femoral bone, and maintain harmonious loading. Hip resurfacing can be indicated for young and/or active subjects for whom restoration of hip biomechanics offers a real advantage over conventional hip arthroplasty. The mid-term results have been encouraging, both clinically and radiographically.

Changes of the bone mineral density in proximal femur following total hip resurfacing arthroplasty in osteonecrosis of femoral head

Total hip resurfacing arthroplasty (THRA) is being performed with increasing frequency for osteonecrosis of femoral head (ONFH). To evaluate femoral bone remodeling in ONFH after THRA and determine the impact of stem-neck angle (SNA) of inserted femoral component on bone remodeling, we monitored the changes in BMD in proximal femur in 23 patients with ONFH after surgery. Patients were divided into group A (SNA >or= 5 degrees ) and group B (SNA < 5 degrees ). The BMD was measured in seven Gruen zones and two neck zones using dual-energy X-ray absorptiometry preoperatively, then at 3, 6, 12, and 24 months after surgery. At all ROIs, the BMD decreased significantly by 3 months postoperatively. The BMD ceased to decrease and reversed by 6 months. The BMD in neck increased significantly in group A, compared with group B at 24 months. The BMD increased 2% at ROI1 at 24 months in both groups, and at ROI7, the BMD in group A reversed to baseline value by 6 months and increased 5.81% at 24 months. These findings implied that the bone stock of proximal femur in ONFH can be well reserved after total hip resurfacing arthroplasty with valgus positioning of the femoral component.

Metal-on-metal hip resurfacing arthroplasty

The main advantage of hip resurfacing is bone conservation for patients likely to outlive a primary conventional hip replacement. Previous attempts at hip resurfacing failed predominantly because of the consequences of a high amount of wear of thin polyethylene acetabular components and poor femoral component fixation. With correct patient selection, surgeon education, and operative technique, survivorship at five years is comparable with that of traditional hip replacements. Hip resurfacing has its own unique set of complications, including a fractured neck of the femur. It is necessary to understand the risk factors prior to performing the procedure.

Metal-on-metal hip resurfacing: a skeptic's view

Contemporary metal-on-metal hip resurfacing is the third attempt by its proponents to eliminate a diaphyseal femoral component. I have multiple objections with the resurfacing concept and believe even the premises for the use of resurfacing invalid. There is a high rate of success with circumferential bead or mesh-coated uncemented stemmed femoral components at 10 to 20 years and there have been no long-term adverse consequences of femoral stress shielding with a diaphyseal component. More acetabular bone may be removed with resurfacing, negating its "conservative" premise. One computer simulation suggested the range of hip motion might be considerably less with resurfacing compared with conventional hip arthroplasty. There are a very limited number of patients for whom hip resurfacing is truly indicated, and the femoral head may be unsuitable for resurfacing in 40% of selected patients. Resurfacing is technically more difficult than conventional hip arthroplasty. Early complications and revision for femoral neck fractures are more likely with resurfacing. Blood and urine metal ion levels, capsular lymphocytic aggregation, and hypersensitivity are concerns with metal-on-metal articulation. Metal-on-metal hip resurfacing should only be used by a limited number of hip surgeons. The risks and complications of metal-on-metal hip resurfacing outweigh any possible advantages.

Hip resurfacing femoral neck fracture influenced by valgus placement

Femoral neck fracture is the most common short-term concern after hip resurfacing arthroplasty. Currently, there is little basis to decide between neutral and valgus placement. We loaded 10 notched cadaveric femur pairs to failure; one side was implanted at 0 degrees relative to the femoral neck and the other at 10 degrees valgus. All 20 were dual-energy X-ray absorptiometry-scanned. Failure load correlated with bone mineral density. Valgus placement increased the fracture load by an average of 28% over neutral for specimens with normal bone mineral density but had no effect on fracture load in specimens with low bone mineral density. For specimens with normal bone mineral density (typical of patients undergoing resurfacing arthroplasty), neutral-valgus placement had a greater effect than bone mineral density, explaining 54% of the fracture load variance. Component placement greater than 10 degrees valgus is likely undesirable because this can lead to an increase in component size and a greater likelihood of notching. To reduce fracture risk, we recommend placing the femoral component in valgus and selecting patients with higher bone mineral density.

Effect of changing indications and techniques on total hip resurfacing

Recently, improved metal-on-metal bearing technology has led to the reemergence of resurfacing as a reasonable option for total hip arthroplasty. During the course of a prospective multicenter FDA-IDE evaluation of metal-on-metal total hip resurfacings, we modified our indications and emphasized surgical technique where the femoral surface area was small due to femoral cysts and small component size. We assessed the influence of these changes on complication rates in the first cohort of 292 patients and the second of 724, and then compared these outcomes in the second cohort with historical reports of resurfacing. We had a minimum followup of 24 months (mean, 33 months; range, 24-60 months). After changes were made in the indications and technique, the overall complication rate decreased from 13.4% to 2.1% with the femoral neck fracture rate reduced from 7.2% to 0.8%. The outcomes of the second cohort compare with modern-day resurfacing devices and appear superior to historical results. The data suggest patients should be carefully selected and technique optimized to reduce complications. Long-term followup is required to see if these promising results will be maintained.

Metal-on-metal hip resurfacing for obese patients

BACKGROUND

The effect of obesity on the outcomes of metal-on-metal resurfacing arthroplasty is not currently known. In this study, we assessed the influence of body mass index on the survival of a metal-on-metal hybrid hip resurfacing prosthesis by comparing the clinical results of patients with a body mass index of >or=30 with those of patients with a body mass index of <30.

METHODS

We retrospectively reviewed our registry to identify all patients who had been followed for at least two years after a metal-on-metal hip resurfacing arthroplasty, and we divided those patients according to whether they had had a body mass index of >or=30 (the study group) or <30 (the control group) at the time of the surgery. One hundred and twenty-five patients (144 hips) with an average weight of 104.6 kg and an average body mass index of 33.4 were included in the study group, and 531 patients (626 hips) with an average weight of 78.3 kg and an average body mass index of 25.4 were included in the control group. We compared the clinical results (UCLA [University of California at Los Angeles] and Harris hip scores, SF-12 [Short Form-12] survey results, and complication rates), radiographic results, and prosthetic survival rates of the two groups.

RESULTS

There was no significant difference postoperatively between the groups with regard to the UCLA pain or walking scores or the mental component score of the SF-12. However, the UCLA function and activity scores were lower in the study group than in the control group (9.2 compared with 9.6 points [p = 0.001] and 7.1 compared with 7.6 points [p = 0.002], respectively). The control group had a significantly higher postoperative physical component score on the SF-12 (51.4 points compared with 49.3 points in the study group, p = 0.01) and postoperative Harris hip score (93.8 compared with 90.6 points, p = 0.0003). Two hips (1.4%) were revised in the study group. In contrast, thirty-one hips (5.0%) were converted to a total hip replacement in the control group; twenty of the thirty-one were revised because of loosening of the femoral component. The five-year survivorship of the hip prostheses was 98.6% in the study group and 93.6% in the control group (p = 0.0401). When the entire cohort was divided into three groups according to whether the body mass index was <25, 25 to 29, or >or=30, the risk of revision was found to have decreased twofold from one group to the next as the body mass index increased (p = 0.013). No acetabular component loosened in either group. The average diameter of the femoral component was 48.3 mm in the study group and 46.8 mm in the control group (p = 0.0001). There were no revisions for any reason and no radiolucencies were observed in a subset of twenty-seven patients with a body mass index of >or=35.

CONCLUSIONS

Metal-on-metal resurfacing hip arthroplasty is performing well in patients with a high body mass index, although the function scores are reduced compared with those for patients with a body mass index of <30. The protective effect of a high body mass index on survivorship results may be explained by a reduced activity level and a greater component size in this patient population.

The operation of the century: total hip replacement

In the 1960s, total hip replacement revolutionised management of elderly patients crippled with arthritis, with very good long-term results. Today, young patients present for hip-replacement surgery hoping to restore their quality of life, which typically includes physically demanding activities. Advances in bioengineering technology have driven development of hip prostheses. Both cemented and uncemented hips can provide durable fixation. Better materials and design have allowed use of large-bore bearings, which provide an increased range of motion with enhanced stability and very low wear. Minimally invasive surgery limits soft-tissue damage and facilitates accelerated discharge and rehabilitation. Short-term objectives must not compromise long-term performance. Computer-assisted surgery will contribute to reproducible and accurate placement of implants. Universal economic constraints in healthcare services dictate that further developments in total hip replacement will be governed by their cost-effectiveness.

Navigation reduces the learning curve in resurfacing total hip arthroplasty

Hip resurfacing is a novel technique with a substantial learning curve resulting in poor outcomes for many patients. We asked whether navigation would influence this learning curve and accuracy of implantation. Twenty medical students earning their degree in surgical technology participated in a randomized trial. We provided instruction about the surgical technique, including the use of conventional instrumentation, the use of a computed tomography-based planner for hip resurfacing, and a navigation system. The 20 students were then split into three groups undertaking these tasks in three different orders. Synthetic femurs replicated normal, osteoarthritis, slipped capital femoral epiphysis, and coxa valga. The mean error using the conventional method to insert a guidewire was 23 degrees; using the computed tomography plan method it was 22 degrees; and using navigation was 7 degrees. Students produced similar accuracy, even in their first attempt, on difficult anatomy when provided navigation. Motivated students rapidly achieved an expert level of accuracy when provided with navigation. Learning a conventional method first did not improve performance, even in difficult cases. Our data suggest navigation may play an important role in reducing the learning curve in hip resurfacing arthroplasty and other tasks in arthroplasty in which a high degree of accuracy is clinically important.

Femoral neck fractures after metal-on-metal total hip resurfacing: a prospective cohort study

There has been a renewed interest in metal-on-metal resurfacing total hip arthroplasty. Recent studies have reported high success rates at short to midterm follow-up. Despite these excellent early outcomes, femoral neck fractures have been reported as a major complication after this procedure. The purpose of this study was to identify the incidence of this complication in a prospective cohort of patients. In addition, various demographic and radiographic factors such as surgeon experience, age, sex, body mass index, femoral neck notching, and cysts were assessed as potential risk factors. Between November 2000 and August 2006, 550 metal-on-metal total hip resurfacings were performed by a single surgeon. The absolute risk for femoral neck fracture in this cohort was 2.5%. Of the 14 fractures, 12 occurred in the first 69 resurfacings performed. After this time, the incidence of fracture was 0.4%. Women and obese patients were shown to have higher cumulative incidences of fractures. These findings suggest the need for careful patient selection and surgical technique, especially for surgeons during the early learning curve for this technically difficult surgery.

Resurfacing THA for patients younger than 50 year: results of 2- to 9-year followup

Resurfacing THA is an attractive treatment option for young patients because it saves the femoral head and is easily revisable. We assessed the clinical outcome of metal-on-metal hybrid total hip resurfacing for the treatment of 295 patients (350 hips) younger than 50 years of age (average, 41.2 years) and compared these results with those of patients 50 years or older (average, 57.4 years) at the time of surgery who were implanted with the same design. Seventy-five percent of the patients were male. The minimum followup was 2 years (mean 5.5 years; range 2-9 years). UCLA hip scores improved to 9.4 for pain; 9.6 for walking; 9.5 for function; and 7.6 for activity. We found no differences in survivorship between younger and older patients, and postoperative clinical scores related to the patients' physical health were comparable. There was no acetabular component loosening. Ten hips (2.8%) were revised for femoral aseptic loosening and one for femoral neck fracture. The 5-year survivorship of hips with good bone quality was 97.8%. There has been no femoral component loosening when the femoral stems were cemented irrespective of bone quality. Metal-on-metal resurfacing THA performs well at short to mid-term followup in young, active adults despite high activity levels.

Osteonecrosis of the femoral head: etiology, imaging and treatment

Osteonecrosis of the femoral head is a disabling clinical entity affecting young adults that usually leads to destruction of the hip joint. A high index of suspicion is necessary for the diagnosis due to the insidious onset of the bone infarcts and the lack of specific clinical signs at the early stages. Many etiology-associated factors have been identified reducing thus the number of idiopathic cases. A number of joint salvaging treatment options are available if early diagnosis can be achieved. MR imaging has been proved to be a highly accurate method both for early diagnosis and for staging of the disease. Replacement of the hip joint is the last resort for pain relief and function, although non-desirable because of the young age of the affected population.

The effects of technique changes on aseptic loosening of the femoral component in hip resurfacing. Results of 600 Conserve Plus with a 3 to 9 year follow-up

The purpose of the present study was to determine the effectiveness of modifications in the surgical technique on loosening of the femoral component in the first 600 consecutive Conserve Plus metal-on-metal hybrid resurfacings (Wright Medical Technologies, Arlington, Tenn). These modifications were gradually introduced over time, but all the changes were implemented after the first 300 hips. The average age of the patients was 48.9 years, and 74% were male. The average follow-up was 70.5 months for the first 300 hips and 42.4 months for the second 300, and there was a significant improvement (P = .016) of the second 300 hips over the first 300 in a time-dependent analysis using as an end point the time to appearance of a radiolucency, suggesting potential femoral component loosening. None of the components with cemented stems showed femoral radiolucencies or were revised for aseptic loosening. Adding fixation holes in the dome and chamfered areas and cleansing and drying using a suction tip in the dome hole were significantly related to the improvement of the results. Positioning the femoral component in a more valgus position did not show any effect as an independent variable.

The accuracy of image-free computer navigation in the placement of the femoral component of the Birmingham Hip Resurfacing

A cadaver study using six pairs of lower limbs was conducted to investigate the accuracy of computer navigation and standard instrumentation for the placement of the Birmingham Hip Resurfacing femoral component. The aim was to place all the femoral components with a stem-shaft angle of 135°.

The mean stem-shaft angle obtained in the standard instrumentation group was 127.7° (120° to 132°), compared with 133.3° (131° to 139°) in the computer navigation group (p = 0.03). The scatter obtained with computer-assisted navigation was approximately half that found using the conventional jig.

Computer navigation was more accurate and more consistent in its placement of the femoral component than standard instrumentation. We suggest that image-free computer-assisted navigation may have an application in aligning the femoral component during hip resurfacing.

Femoral head blood flow during hip resurfacing

Preserving femoral head vascularity during hip resurfacing may avoid femoral neck fractures and late femoral loosening. The posterior approach and notching of the femoral neck influence femoral head perfusion. However, it is not known if standard preparation of the femoral head during hip resurfacing can disrupt blood flow. Ten patients (10 hips) with advanced osteoarthritis having metal-on-metal hip resurfacing by means of a vascular-preserving surgical approach had femoral head blood flow measurements using laser Doppler flowmetry. Nine hips had a mean decrease of 70% in femoral head blood flow after standard reaming and preparation. The data suggest femoral head reaming during hip resurfacing substantially impacts blood flow to the femoral head and infers the extra osseous blood supply is still important in the arthritic femoral head. To avoid damaging the retinacular vessels, surgeons should direct the cylindrical reamer superolaterally staying as close as possible to the inferomedial neck.

Results of metal-on-metal hybrid hip resurfacing for Crowe type-I and II developmental dysplasia

BACKGROUND

Modern hip resurfacing implants may increase stability and preserve more bone than conventional total hip arthroplasty. The purpose of this retrospective study was to analyze the mid-term results in a consecutive series of middle-aged patients with developmental dysplasia of the hip treated with hybrid resurfacing joint arthroplasty.

METHODS

Metal-on-metal hip resurfacing was performed in fifty-one patients (fifty-nine hips), forty-two of whom were female and nine of whom were male. The average age at the time of surgery was 43.7 years. Radiographic and clinical data were collected at six weeks, at three months, and at yearly follow-up visits. Seven hips had Crowe type-II developmental dysplasia of the hip and fifty-two had type-I.

RESULTS

The follow-up period ranged from 4.2 to 9.5 years (average, 6.0 years). Initial stability was achieved in all but three hips. The clinical outcomes, as rated with the University of California at Los Angeles (UCLA) hip score, improved significantly compared with the preoperative ratings. On the average, the pain rating improved from 3.2 to 9.3 points; the score for walking, from 6.0 to 9.7 points; the score for function, from 5.7 to 9.6 points; and the score for activity, from 4.6 to 7.3 points (all p = 0.0001). The mean Short Form-12 (SF-12) mental score increased from 46.6 to 53.5 points, and the mean SF-12 physical score increased from 31.7 to 51.4 points (both p < 0.0001). The mean postoperative Harris hip score was 92.5 points. On the average, the range of flexion improved from 106 degrees to 129.6 degrees ; the abduction-adduction arc, from 41.9 degrees to 76.9 degrees ; and the rotation arc in extension, from 32.1 degrees to 84.8 degrees (all p = 0.0001). Four patients delivered a total of six healthy babies since the time of implantation of the prosthesis. Radiographic analysis showed a decrease in the mean body weight lever arm from 118.5 mm preoperatively to 103.9 mm postoperatively (p = 0.007). There were five femoral failures requiring conversion to a total hip arthroplasty. One hip showed a radiolucency around the metaphyseal femoral stem. There were no complete acetabular radiolucencies, and all sockets remained well fixed.

CONCLUSIONS

The mid-term results of metal-on-metal resurfacing in patients with Crowe type-I or II developmental dysplasia of the hip were disappointing with respect to the durability of the femoral component. However, the fixation of the porous-coated acetabular components without adjuvant fixation was excellent despite incomplete lateral acetabular coverage of the socket. More rigorous patient selection and especially meticulous bone preparation are essential to minimize femoral neck fractures and loosening after this procedure.

Five-year results of metal-on-metal resurfacing arthroplasty in Asian patients

Clinical results of 50 metal-on-metal resurfacing arthroplasties in 45 Japanese patients were evaluated to a minimum follow-up of 5 years. The predominant diagnosis was developmental dysplasia or dislocation of the hip (70%). One patient died of an unrelated cause and another was lost to follow-up. Two hips received revision surgery, including 1 femoral neck fracture and 1 septic loosening. In the remaining 46 hips, 1 hip showed femoral component loosening. Clinical scores of the 46 hips were satisfactory at the final follow-up. The survival rate at 5 years was 96% when failure was attributed to revision for any reason. Metal-on-metal resurfacing arthroplasty in Japanese patients, who have a different distribution of hip diseases from European and American patients, showed similarly promising early clinical results.

The femoral head/neck offset and hip resurfacing

Because the femoral head/neck junction is preserved in hip resurfacing, patients may be at greater risk of impingement, leading to abnormal wear patterns and pain. We assessed femoral head/neck offset in 63 hips undergoing metal-on-metal hip resurfacing and in 56 hips presenting with non-arthritic pain secondary to femoroacetabular impingement. Most hips undergoing resurfacing (57%; 36) had an offset ratio ≤ 0.15 pre-operatively and required greater correction of offset at operation than the rest of the group. In the non-arthritic hips the mean offset ratio was 0.137 (0.04 to 0.23), with the offset ratio correlating negatively to an increasing α angle. An offset ratio ≤ 0.15 had a 9.5-fold increased relative risk of having an α angle ≥ 50.5°. Most hips undergoing resurfacing have an abnormal femoral head/neck offset, which is best assessed in the sagittal plane.

Surgical variables affect the mechanics of a hip resurfacing system

Recent clinical studies have linked failure to surgical variables of stemmed hip resurfacing systems. We used finite element analysis to investigate the effects of implant position, stem orientation, and extent of fixation both on the local stresses and strains associated with implant loosening, neck fracture, and stem fracture, as well as on the load transfer distribution in the bone-implant system. The range of peak stress in the cement was reduced from 11 to 13 MPa for the varus stem to 3.2 to 4.2 MPa for the valgus stem. The range of peak strain in the bone was also reduced from -0.35% to -0.45% strain for the varus stem to -0.19% to -0.27% strain for the valgus stem, but only when reamed cancellous bone remained exposed. Peak stresses in the stem were low for all cases. Additionally, the implant's load transfer distribution was generally insensitive to all variables examined and the femoral head was substantially unloaded. Our data indicate the local stresses and strains associated with implant loosening and neck fracture were reduced by placing the implant in a valgus orientation and covering reamed cancellous bone, but unloading of the femoral head, found for all variables examined, may lead to adverse bone remodeling.