The Journal of Arthroplasty 30 (2015) 1799–1803
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Comparison of Acetabular Bone Resection, Offset, Leg Length and Post Operative Function Between Hip Resurfacing Arthroplasty and Total Hip Arthroplasty Michael C. Parry, BSc, MBChB, MD, FRCS , James Povey, BSc, Ashley W. Blom, MD, PhD, FRCS, Michael R. Whitehouse, BSc, MBChB, MD, PhD, FRCS Muculoskeletal Research Unit, University of Bristol, Level 1 Learning and Research Building, Southmead Hospital, Westbury-on-Trym, Bristol, UK
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Article history: Received 7 January 2015 Accepted 20 April 2015 Keywords: hip resurfacing arthroplasty total hip arthroplasty primary hip arthroplasty acetabular resection ceramic on ceramic bearing
a b s t r a c t Controversy exists regarding the amount of acetabular bone resection, biomechanics and function of patients receiving either total hip arthroplasty (THA) or hip resurfacing arthroplasty (HRA). A cohort of patients undergoing 36 mm ceramic-on-ceramic THA (89) or metal-on-metal HRA (86) were compared. No difference was observed when the ratio of native femoral head size was compared to the implanted acetabular component size (1.15±0.1 HRA c.f. 1.13±0.1 THA). No difference was observed in acetabular offset, vertical centre of rotation or function (OHS mean 47 in both groups) but leg length discrepancy (1.8 mm c.f. 5.5 mm) and femoral offset did differ (0.6 mm c.f. 4.1 mm). This demonstrates that 36 mm ceramic-on-ceramic THA is not associated with more bone resection than HRA and achieves equivalent function whilst avoiding the problems of metal-onmetal bearings. © 2015 Elsevier Inc. All rights reserved.
Total hip arthroplasty (THA) is an effective intervention for the treatment of end stage arthritis of the hip, demonstrating both cost effectiveness [1,2] and marked improvement in quality of life [3]. However, implant survivorship is disappointing in younger patients with failure rates of between 10 and 20% at 10 years in patients younger than 60 years [4–7]. Hip resurfacing arthroplasty (HRA) appeared to be an attractive option in younger patients as metal bearings had favourable tribology and resurfacing was thought to preserve bone stock. Results using large diameter metal on metal HRA designs from designer surgeons were encouraging with failure rates as low as 0.02% at 3 years and 4.5% at 10 years [8,9] which was attributed to improved biomechanics, preservation of bone and reduced wear rates seen with large diameter components [10–13]. Whilst some have reported improved outcomes for HRA when compared to THA [14] in non-randomised studies, results from a randomised control trial failed to demonstrate a benefit of either design [15]. Registry data has shown that the results obtained by the generality of surgeons do not replicate the results of selected series, with higher failure rates associated with the use of resurfacing in the majority of patients, the exception being men with One or more of the authors of this paper have disclosed potential or pertinent conflicts of interest, which may include receipt of payment, either direct or indirect, institutional support, or association with an entity in the biomedical field which may be perceived to have potential conflict of interest with this work. For full disclosure statements refer to http://dx.doi.org/10.1016/j.arth.2015.04.030. Reprint requests: Michael C Parry, BSc, MBChB, MD, FRCS, Muculoskeletal Research Unit, University of Bristol, Level 1 Learning and Research Building, Southmead Hospital, Westbury-on-Trym, Bristol, UK, BS10 5NB. http://dx.doi.org/10.1016/j.arth.2015.04.030 0883-5403/© 2015 Elsevier Inc. All rights reserved.
large femoral heads [16,17]. Stemmed metal-on-metal implants have even higher failure rates [18]. These early failures are attributable in part to the adverse reactions to metal debris seen in a number of patients [19]. Accurate restoration of normal anatomy is important as it effects leg length, femoral and acetabular offset, post-operative gait, trochanteric pain, instability and wear [20–23]. Maintaining bone stock at primary surgery is particularly important in younger patients as it facilitates revision surgery. HRA resects less bone than THA on the femoral side [24]. Reports in the literature of bone resection on the acetabular side vary with some authors suggesting that bone resection is equivalent between HRA and THA [11,25], some suggesting increased bone removal in HRA [26–28] and one group suggesting increased bone resection in THA [29]. Not all of the studies accounted for the size of the native acetabulum (or femoral head) and the components used in the comparator THA groups were sometimes out-dated and no longer in widespread use [13]. The primary objective of this study was to determine whether HRA or THA preserved more acetabular bone. We used acetabular implant size as an indirect measure of resected bone, but controlled for femoral head size as larger hips would require larger components. We achieved this by simply comparing the ratio of native femoral head size to implanted acetabular component size. Secondary objectives were to compare leg length discrepancy, acetabular and femoral offset, and restoration of vertical centre of rotation in HRA and THA and to determine whether these variables correlated with post-operative patient reported outcomes.
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Patients and Methods The study comprised a retrospective analysis of patients identified from the institutional database on whom either a HRA or conventional THA were performed between January 2004 and December 2010. All patients with a minimum 2-year follow up were considered for inclusion. The HRA group comprised patients who underwent hip resurfacing using the Birmingham Hip Resurfacing (Smith and Nephew, Memphis, TN, USA) whilst the THA group comprised patients who had undergone hip arthroplasty with an uncemented Corail femoral implant and a Pinnacle acetabular component (DePuy Int., Leeds, UK) utilizing a 36 mm ceramic on ceramic articulation. During this period, 101 HRA and 92 THA procedures of these designs were performed. All procedures were performed through a posterior approach by a consultant or senior trainee under direct supervision. The dissection required for each procedure was comparable though the HRA necessitated a more extensive capsular release. There was no difference in the method of reduction between the two groups. The method of anaesthesia was at the discretion of the anaesthetist responsible for each case. However, during this period, the conventional method of anaesthesia was by spinal anaesthetic and sedation, except in exceptional circumstances, for both THA and HRA. Data collected included patient age, gender, acetabular shell size, femoral component size, prosthetic head size, pre-operative femoral head size, and maximum acetabular reamer size used intraoperatively. Patients were excluded on the grounds of insufficient pre- or postoperative radiographs or revision within 2 years of the index procedure, resulting in a final study population of 86 patients in the HRA group and 89 cases in the THA group. Radiological Assessment This was performed on both preoperative and postoperative standardized radiographs of the pelvis with knees extended and hips internally rotated [23] using freely available image analysis software (www. imatri.net/medical). Postoperative radiographs were calibrated using
the known diameter of the implanted femoral head. Once the radiograph was calibrated, the contralateral femoral head diameter was measured and this was used to calibrate the preoperative radiograph. Measurements were recorded using standardised anatomical landmarks [23,30]. Leg length was calculated using the method of Konyves and Bannister [23]. Shortening of the leg length was assigned a negative value whilst increases in leg length were assigned positive values. Femoral offset was measured as the perpendicular distance between the centre of rotation of the femoral head and a reference line drawn through the centre of the medullary canal. Acetabular offset was measured as the distance between the centre of rotation of the femoral head and a vertical reference line drawn through the centre of the acetabular teardrop. The sum of these two values was defined as the global offset. The vertical centre of rotation was defined as the distance between the centre of femoral rotation and a line drawn between the most distal aspect of both acetabular teardrops, with increase in offset assigned positive values and reductions assigned negative values (Fig. 1). Measurements of Functional Outcome Patient reported outcomes at a minimum 2 years post operation were recorded by means of the Oxford Hip Score (OHS) [31], a 12 item questionnaire with an improving score range from 0–48, the short form-12 (SF-12) physical and mental questionnaire [32], and a self-administered patient satisfaction [33] which assesses satisfaction in four domains following surgery, scoring from 25 to 100. Statistical Analysis Statistical analysis comprised an initial assessment of normality using the D’Agostini-Pearson test. Where normally distributed, comparisons between the two groups were assessed using an unpaired two-tail t-test with data presented as means and standard deviation. For nonnormally distributed data, a Mann–Whitney U-test was used and data presented as median and inter-quartile range. Assessment of
Fig. 1. Radiographic measurements based on anatomical landmarks. Leg length difference was calculated as the addition of the distance between a line drawn intersecting the acetabular teardrop and the most medial visible point on the lesser trochanter (B + D) subtracted from the contralateral limb (A + C). The femoral offset (E) was measured as the distance between a line subtending the long axis of the femur and the centre of rotation of the hip. The acetabular offset (F) was measured as the distance between the centre of rotation and a vertical line intersecting the acetabular teardrop. The vertical centre of rotation (VCOR) was measured as the distance between the centre of rotation and the horizontal line intersecting the acetabular teardrop (B and D).
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association between independent factors was made using a multiple regression analysis. All analysis was completed using GraphPad Prism version 6.0 (GraphPad Software, La Jolla, CA, USA). Statistical significance was considered with a p-value less than 0.05. Results The HRA population comprised 87 patients, of which 79 were male, with a mean age at operation of 53 (range 32–66) and a mean follow up of 6 years (range 3.4–9.6). The THA group comprised 89 patients, of which 34 were males, with a mean age of 58 (range 36–77) and a mean follow up of 5 years (range 3.4–8.3). A significant difference in age (P = 0.0002) and gender distribution (P b 0.0001) was noted between the groups. One patient in the THA group required a 2-stage revision for infection, whilst 4 in the HRA underwent revision (two cases for periprosthetic femoral neck fractures, one case for adverse reaction to metal debris and one case for dislocation). Two patients in the THA group underwent reoperation other than revision (one case underwent a radical debridement for early postoperative infection, the second case required removal of a cable placed at primary surgery due to an undisplaced medial calcar crack), which compared to none in the HRA group. Acetabular Bone Resection The mean size of the acetabular component in the HRA group was 56 mm (SD 2.6), which compares to the mean of 52 mm (SD 2.6) seen in the THA group (P b 0.001) (Fig. 2). However, the mean native femoral head size in the HRA group was also larger than that of the THA group (50.3 mm (SD 3.4) c.f. 46.6 (SD 4.0), P b 0.001). Thus, there was no statistical difference in the mean ratio of native femoral head size to acetabular component between the groups (1.15 (SD 0.1) in the HRA group c.f. 1.13 (SD 0.1) in the THA group, P = 0.054). Radiographic Outcomes The mean pre operative femoral offset in the THA group was 35.5 mm (SD 8.3), which was increased to 39.6 mm (SD 6.6) post operation (P b 0.001). In the HRA group, the pre operative femoral offset was 37.7 mm (SD 6.7), which was reduced to 37.3 mm (SD 6.2) post operation (P = 0.660). THA led to a significantly greater increase when
Diameter (mm)
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compared to the HRA group (4.1 mm (SD 7.2) c.f. 0.4 mm (SD 5.4), P b 0.001). The mean acetabular offset in the THA group was 34.8 mm (SD 5.7), which was reduced to 31.0 mm (SD 3.8) post operation (P b 0.001). In the HRA group, the mean acetabular offset pre operation was 37.1 mm (SD 4.8), which was reduced to 33.4 mm (SD 3.7) post operation (P b 0.001). No significant difference was demonstrated between the groups when comparing changes in acetabular offset (THA − 3.8 mm (SD 5.5) c.f. HRA −3.7 mm (SD 4.7); P = 0.946). The pre operative VCOR in the THA group was 14.8 mm (SD 5.7), which increased to 15.9 mm (SD 4.7) post operation (P = 0.175). In the HRA group, the mean pre operative VCOR was 14.4 mm (SD 3.6), which increased to 16.1 mm (SD 4.1) post operation (P = 0.004). No significant difference was demonstrated between the groups when comparing changes in VCOR (THA 1.1 mm (SD 4.4) c.f. HRA 1.7 mm (SD 4.1); P = 0.314). The mean pre operative leg length, as measured from the centre of rotation to the most medial point of the lesser trochanter, in the THA group was 39.0 mm (SD 7.8), which increased to a mean of 44.6 mm (SD 7.2) post operation (P b 0.001). In the HRA group the mean pre operative leg length was 40.7 mm (SD 6.8), which increased to a mean of 42.5 mm (SD 6.9) post operation (P = 0.086). THA significantly increased leg length when compared to HRA (THA 5.6 mm (SD 5.9) c.f. HRA 1.8 mm (SD 5.3); P b 0.001).
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The mean post-operative OHS in both groups were comparable (47 in both groups, P = 0.33). There were also no significant differences between the THA group and HRA group in either the mental (53.0 c.f. 54.7, P = 0.53) or physical (52.5 c.f. 51.8 P = 0.69) components of the SF-12. There was no significant difference in the SAPS between the two groups (100.0 c.f. 100.0, P = 0.95) (Fig. 3).
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Fig. 3. Patient reported outcome measures following either THA or HRA for each of the patient reported outcome scores. Each bar represents the median scores whilst the whiskers represent interquartile ranges.
Fig. 2. The mean size and standard deviation of native femoral head and acetabular component in the two groups.
Multiple regression analysis revealed that neither age, post operative femoral or acetabular offset, leg length nor VCOR were identified as independent predictors of poor functional outcome as depicted by post operative OHS, SF-12 and SAPS. No difference was observed in the ratio of native femoral head size to implanted acetabular component between the 2 implants and further, this ratio was not identified as a predictor of poor functional outcome.
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Discussion This study shows that HRA and THA appear to result in similar resection of acetabular bone when size of the native hip is taken into account. Proponents of HRA cite preservation of bone stock when compared to THA as an advantage of the technique. Preservation of femoral bone stock is undoubtedly achieved [24]. The size of the acetabular component however is directly related to the femoral component in HRA, so larger native femoral heads necessitate removing more acetabular bone. Controversy exists regarding the amount of bone removed from the acetabulum during resurfacing [26,28,29,34–36], though comparison of these studies is difficult due to methodological limitations. Whereas previous studies failed to control for anatomical variation [34,36], or used a spectrum of implants and sizes [26,28] and a variety of implant techniques [26,34,35], we aimed to control for these variables through referencing from existing anatomical markers and using only one design of each component with a standardised head size and bearing surface in the THA group that is suitable for young, active and high demand patients who may otherwise be considered for HRA. It is feasible that in a minority of cases, surgeons may have elected to upsize the acetabular component to accommodate a 36 mm bearing and whilst we cannot accurately assess this number, this may contribute to the lack of difference in bone resection between the two groups. Vendittoli et al. [11] demonstrated no significant difference in the size of acetabular component inserted in either THA or HRA, though in 7% of cases, the acetabular component needed upsizing to accommodate the size of the femoral component in the HRA group. Cross et al. [29] also reported a comparable number of cases requiring upsizing, though it should be noted that, when comparing the ratio of acetabular component to native femoral head size, the authors demonstrated a smaller ratio in the HRA group compared to the THA group. In an attempt to quantify the volume of bone removed through each procedure, Su et al. [37] performed a cadaveric study recording the weight of acetabular reamings from either procedure. Whilst no difference was demonstrated once controlled for the native femoral head, it should be noted that the cadavers were non-arthritic, with normal anatomy, which will affect the volume of bone and cartilage resected. Brennan et al. [25] failed to demonstrate any difference between the weight of reamings in a population of non-randomised patients receiving either HRA or THA. We have shown that in our cohort THA was worse at restoring femoral offset and leg length. However, a number of authors have found the opposite [30,38,39]. In certain circumstances, this ability to vary offset and length may be desirable, for example in the case of significant pre-operative shortening or lack of soft tissue tension. The limitations of each design to adjust these variables should be considered during pre-operative planning. It should be noted that the changes in offset and leg lengths in our study were very small, in the order of millimetres, and thus although numerically statistically significant may not necessarily be of any clinical significance. Indeed previous studies have shown that mild radiological changes correlate poorly with clinical outcomes [40] and in particular perception of leg length discrepancy and true leg length inequality correlate poorly [41]. It is thus perhaps unsurprising that although there were radiological differences in our two groups there were no differences in clinical scores. We did not compare this change in leg length with the pre-operative leg length discrepancy but rather looked only at the change in length of the operated leg. Whilst this is a potential limitation of this study in that we have not addressed the issue of restoration of anatomy, rather looked only at a change in anatomy, this does highlight the limitation of HRA to significantly address leg length discrepancy. This finding is consistent with others who have also showed no differences in patient reported outcomes between THA and HRA in both cohort studies [42–45] and randomised controlled trials [15]. Patient reported outcomes tend to be dominated by measures of pain and some
authors have demonstrated better objectively measured pure functional outcomes following HRA in comparison to THA [14,35,46–51]. Whilst our study differs in design to many of those previously presented, and suffers from the same drawbacks as many of these previous studies (non-randomised design, retrospective cohort analysis and nonmatched patient populations), our findings agree with those of others in that despite variations in restoration of hip mechanics and anatomy, this does not translate into a difference in functional or patient reported outcome according to contemporary available outcome measures. Conclusion The amount of bone resected from the acetabulum (as measured by the size of acetabular implant) does not differ between hip resurfacing arthroplasty and total hip arthroplasty. 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The Journal of Arthroplasty journal homepage: www.arthroplastyjournal.org
Comparison of Acetabular Bone Resection, Offset, Leg Length and Post Operative Function Between Hip Resurfacing Arthroplasty and Total Hip Arthroplasty Michael C. Parry, BSc, MBChB, MD, FRCS , James Povey, BSc, Ashley W. Blom, MD, PhD, FRCS, Michael R. Whitehouse, BSc, MBChB, MD, PhD, FRCS Muculoskeletal Research Unit, University of Bristol, Level 1 Learning and Research Building, Southmead Hospital, Westbury-on-Trym, Bristol, UK
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Article history: Received 7 January 2015 Accepted 20 April 2015 Keywords: hip resurfacing arthroplasty total hip arthroplasty primary hip arthroplasty acetabular resection ceramic on ceramic bearing
a b s t r a c t Controversy exists regarding the amount of acetabular bone resection, biomechanics and function of patients receiving either total hip arthroplasty (THA) or hip resurfacing arthroplasty (HRA). A cohort of patients undergoing 36 mm ceramic-on-ceramic THA (89) or metal-on-metal HRA (86) were compared. No difference was observed when the ratio of native femoral head size was compared to the implanted acetabular component size (1.15±0.1 HRA c.f. 1.13±0.1 THA). No difference was observed in acetabular offset, vertical centre of rotation or function (OHS mean 47 in both groups) but leg length discrepancy (1.8 mm c.f. 5.5 mm) and femoral offset did differ (0.6 mm c.f. 4.1 mm). This demonstrates that 36 mm ceramic-on-ceramic THA is not associated with more bone resection than HRA and achieves equivalent function whilst avoiding the problems of metal-onmetal bearings. © 2015 Elsevier Inc. All rights reserved.
Total hip arthroplasty (THA) is an effective intervention for the treatment of end stage arthritis of the hip, demonstrating both cost effectiveness [1,2] and marked improvement in quality of life [3]. However, implant survivorship is disappointing in younger patients with failure rates of between 10 and 20% at 10 years in patients younger than 60 years [4–7]. Hip resurfacing arthroplasty (HRA) appeared to be an attractive option in younger patients as metal bearings had favourable tribology and resurfacing was thought to preserve bone stock. Results using large diameter metal on metal HRA designs from designer surgeons were encouraging with failure rates as low as 0.02% at 3 years and 4.5% at 10 years [8,9] which was attributed to improved biomechanics, preservation of bone and reduced wear rates seen with large diameter components [10–13]. Whilst some have reported improved outcomes for HRA when compared to THA [14] in non-randomised studies, results from a randomised control trial failed to demonstrate a benefit of either design [15]. Registry data has shown that the results obtained by the generality of surgeons do not replicate the results of selected series, with higher failure rates associated with the use of resurfacing in the majority of patients, the exception being men with One or more of the authors of this paper have disclosed potential or pertinent conflicts of interest, which may include receipt of payment, either direct or indirect, institutional support, or association with an entity in the biomedical field which may be perceived to have potential conflict of interest with this work. For full disclosure statements refer to http://dx.doi.org/10.1016/j.arth.2015.04.030. Reprint requests: Michael C Parry, BSc, MBChB, MD, FRCS, Muculoskeletal Research Unit, University of Bristol, Level 1 Learning and Research Building, Southmead Hospital, Westbury-on-Trym, Bristol, UK, BS10 5NB. http://dx.doi.org/10.1016/j.arth.2015.04.030 0883-5403/© 2015 Elsevier Inc. All rights reserved.
large femoral heads [16,17]. Stemmed metal-on-metal implants have even higher failure rates [18]. These early failures are attributable in part to the adverse reactions to metal debris seen in a number of patients [19]. Accurate restoration of normal anatomy is important as it effects leg length, femoral and acetabular offset, post-operative gait, trochanteric pain, instability and wear [20–23]. Maintaining bone stock at primary surgery is particularly important in younger patients as it facilitates revision surgery. HRA resects less bone than THA on the femoral side [24]. Reports in the literature of bone resection on the acetabular side vary with some authors suggesting that bone resection is equivalent between HRA and THA [11,25], some suggesting increased bone removal in HRA [26–28] and one group suggesting increased bone resection in THA [29]. Not all of the studies accounted for the size of the native acetabulum (or femoral head) and the components used in the comparator THA groups were sometimes out-dated and no longer in widespread use [13]. The primary objective of this study was to determine whether HRA or THA preserved more acetabular bone. We used acetabular implant size as an indirect measure of resected bone, but controlled for femoral head size as larger hips would require larger components. We achieved this by simply comparing the ratio of native femoral head size to implanted acetabular component size. Secondary objectives were to compare leg length discrepancy, acetabular and femoral offset, and restoration of vertical centre of rotation in HRA and THA and to determine whether these variables correlated with post-operative patient reported outcomes.
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Patients and Methods The study comprised a retrospective analysis of patients identified from the institutional database on whom either a HRA or conventional THA were performed between January 2004 and December 2010. All patients with a minimum 2-year follow up were considered for inclusion. The HRA group comprised patients who underwent hip resurfacing using the Birmingham Hip Resurfacing (Smith and Nephew, Memphis, TN, USA) whilst the THA group comprised patients who had undergone hip arthroplasty with an uncemented Corail femoral implant and a Pinnacle acetabular component (DePuy Int., Leeds, UK) utilizing a 36 mm ceramic on ceramic articulation. During this period, 101 HRA and 92 THA procedures of these designs were performed. All procedures were performed through a posterior approach by a consultant or senior trainee under direct supervision. The dissection required for each procedure was comparable though the HRA necessitated a more extensive capsular release. There was no difference in the method of reduction between the two groups. The method of anaesthesia was at the discretion of the anaesthetist responsible for each case. However, during this period, the conventional method of anaesthesia was by spinal anaesthetic and sedation, except in exceptional circumstances, for both THA and HRA. Data collected included patient age, gender, acetabular shell size, femoral component size, prosthetic head size, pre-operative femoral head size, and maximum acetabular reamer size used intraoperatively. Patients were excluded on the grounds of insufficient pre- or postoperative radiographs or revision within 2 years of the index procedure, resulting in a final study population of 86 patients in the HRA group and 89 cases in the THA group. Radiological Assessment This was performed on both preoperative and postoperative standardized radiographs of the pelvis with knees extended and hips internally rotated [23] using freely available image analysis software (www. imatri.net/medical). Postoperative radiographs were calibrated using
the known diameter of the implanted femoral head. Once the radiograph was calibrated, the contralateral femoral head diameter was measured and this was used to calibrate the preoperative radiograph. Measurements were recorded using standardised anatomical landmarks [23,30]. Leg length was calculated using the method of Konyves and Bannister [23]. Shortening of the leg length was assigned a negative value whilst increases in leg length were assigned positive values. Femoral offset was measured as the perpendicular distance between the centre of rotation of the femoral head and a reference line drawn through the centre of the medullary canal. Acetabular offset was measured as the distance between the centre of rotation of the femoral head and a vertical reference line drawn through the centre of the acetabular teardrop. The sum of these two values was defined as the global offset. The vertical centre of rotation was defined as the distance between the centre of femoral rotation and a line drawn between the most distal aspect of both acetabular teardrops, with increase in offset assigned positive values and reductions assigned negative values (Fig. 1). Measurements of Functional Outcome Patient reported outcomes at a minimum 2 years post operation were recorded by means of the Oxford Hip Score (OHS) [31], a 12 item questionnaire with an improving score range from 0–48, the short form-12 (SF-12) physical and mental questionnaire [32], and a self-administered patient satisfaction [33] which assesses satisfaction in four domains following surgery, scoring from 25 to 100. Statistical Analysis Statistical analysis comprised an initial assessment of normality using the D’Agostini-Pearson test. Where normally distributed, comparisons between the two groups were assessed using an unpaired two-tail t-test with data presented as means and standard deviation. For nonnormally distributed data, a Mann–Whitney U-test was used and data presented as median and inter-quartile range. Assessment of
Fig. 1. Radiographic measurements based on anatomical landmarks. Leg length difference was calculated as the addition of the distance between a line drawn intersecting the acetabular teardrop and the most medial visible point on the lesser trochanter (B + D) subtracted from the contralateral limb (A + C). The femoral offset (E) was measured as the distance between a line subtending the long axis of the femur and the centre of rotation of the hip. The acetabular offset (F) was measured as the distance between the centre of rotation and a vertical line intersecting the acetabular teardrop. The vertical centre of rotation (VCOR) was measured as the distance between the centre of rotation and the horizontal line intersecting the acetabular teardrop (B and D).
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association between independent factors was made using a multiple regression analysis. All analysis was completed using GraphPad Prism version 6.0 (GraphPad Software, La Jolla, CA, USA). Statistical significance was considered with a p-value less than 0.05. Results The HRA population comprised 87 patients, of which 79 were male, with a mean age at operation of 53 (range 32–66) and a mean follow up of 6 years (range 3.4–9.6). The THA group comprised 89 patients, of which 34 were males, with a mean age of 58 (range 36–77) and a mean follow up of 5 years (range 3.4–8.3). A significant difference in age (P = 0.0002) and gender distribution (P b 0.0001) was noted between the groups. One patient in the THA group required a 2-stage revision for infection, whilst 4 in the HRA underwent revision (two cases for periprosthetic femoral neck fractures, one case for adverse reaction to metal debris and one case for dislocation). Two patients in the THA group underwent reoperation other than revision (one case underwent a radical debridement for early postoperative infection, the second case required removal of a cable placed at primary surgery due to an undisplaced medial calcar crack), which compared to none in the HRA group. Acetabular Bone Resection The mean size of the acetabular component in the HRA group was 56 mm (SD 2.6), which compares to the mean of 52 mm (SD 2.6) seen in the THA group (P b 0.001) (Fig. 2). However, the mean native femoral head size in the HRA group was also larger than that of the THA group (50.3 mm (SD 3.4) c.f. 46.6 (SD 4.0), P b 0.001). Thus, there was no statistical difference in the mean ratio of native femoral head size to acetabular component between the groups (1.15 (SD 0.1) in the HRA group c.f. 1.13 (SD 0.1) in the THA group, P = 0.054). Radiographic Outcomes The mean pre operative femoral offset in the THA group was 35.5 mm (SD 8.3), which was increased to 39.6 mm (SD 6.6) post operation (P b 0.001). In the HRA group, the pre operative femoral offset was 37.7 mm (SD 6.7), which was reduced to 37.3 mm (SD 6.2) post operation (P = 0.660). THA led to a significantly greater increase when
Diameter (mm)
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compared to the HRA group (4.1 mm (SD 7.2) c.f. 0.4 mm (SD 5.4), P b 0.001). The mean acetabular offset in the THA group was 34.8 mm (SD 5.7), which was reduced to 31.0 mm (SD 3.8) post operation (P b 0.001). In the HRA group, the mean acetabular offset pre operation was 37.1 mm (SD 4.8), which was reduced to 33.4 mm (SD 3.7) post operation (P b 0.001). No significant difference was demonstrated between the groups when comparing changes in acetabular offset (THA − 3.8 mm (SD 5.5) c.f. HRA −3.7 mm (SD 4.7); P = 0.946). The pre operative VCOR in the THA group was 14.8 mm (SD 5.7), which increased to 15.9 mm (SD 4.7) post operation (P = 0.175). In the HRA group, the mean pre operative VCOR was 14.4 mm (SD 3.6), which increased to 16.1 mm (SD 4.1) post operation (P = 0.004). No significant difference was demonstrated between the groups when comparing changes in VCOR (THA 1.1 mm (SD 4.4) c.f. HRA 1.7 mm (SD 4.1); P = 0.314). The mean pre operative leg length, as measured from the centre of rotation to the most medial point of the lesser trochanter, in the THA group was 39.0 mm (SD 7.8), which increased to a mean of 44.6 mm (SD 7.2) post operation (P b 0.001). In the HRA group the mean pre operative leg length was 40.7 mm (SD 6.8), which increased to a mean of 42.5 mm (SD 6.9) post operation (P = 0.086). THA significantly increased leg length when compared to HRA (THA 5.6 mm (SD 5.9) c.f. HRA 1.8 mm (SD 5.3); P b 0.001).
55 Functional Outcomes
50
The mean post-operative OHS in both groups were comparable (47 in both groups, P = 0.33). There were also no significant differences between the THA group and HRA group in either the mental (53.0 c.f. 54.7, P = 0.53) or physical (52.5 c.f. 51.8 P = 0.69) components of the SF-12. There was no significant difference in the SAPS between the two groups (100.0 c.f. 100.0, P = 0.95) (Fig. 3).
45
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Independent Predictors of Outcome
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iv e
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Fig. 3. Patient reported outcome measures following either THA or HRA for each of the patient reported outcome scores. Each bar represents the median scores whilst the whiskers represent interquartile ranges.
Fig. 2. The mean size and standard deviation of native femoral head and acetabular component in the two groups.
Multiple regression analysis revealed that neither age, post operative femoral or acetabular offset, leg length nor VCOR were identified as independent predictors of poor functional outcome as depicted by post operative OHS, SF-12 and SAPS. No difference was observed in the ratio of native femoral head size to implanted acetabular component between the 2 implants and further, this ratio was not identified as a predictor of poor functional outcome.
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Discussion This study shows that HRA and THA appear to result in similar resection of acetabular bone when size of the native hip is taken into account. Proponents of HRA cite preservation of bone stock when compared to THA as an advantage of the technique. Preservation of femoral bone stock is undoubtedly achieved [24]. The size of the acetabular component however is directly related to the femoral component in HRA, so larger native femoral heads necessitate removing more acetabular bone. Controversy exists regarding the amount of bone removed from the acetabulum during resurfacing [26,28,29,34–36], though comparison of these studies is difficult due to methodological limitations. Whereas previous studies failed to control for anatomical variation [34,36], or used a spectrum of implants and sizes [26,28] and a variety of implant techniques [26,34,35], we aimed to control for these variables through referencing from existing anatomical markers and using only one design of each component with a standardised head size and bearing surface in the THA group that is suitable for young, active and high demand patients who may otherwise be considered for HRA. It is feasible that in a minority of cases, surgeons may have elected to upsize the acetabular component to accommodate a 36 mm bearing and whilst we cannot accurately assess this number, this may contribute to the lack of difference in bone resection between the two groups. Vendittoli et al. [11] demonstrated no significant difference in the size of acetabular component inserted in either THA or HRA, though in 7% of cases, the acetabular component needed upsizing to accommodate the size of the femoral component in the HRA group. Cross et al. [29] also reported a comparable number of cases requiring upsizing, though it should be noted that, when comparing the ratio of acetabular component to native femoral head size, the authors demonstrated a smaller ratio in the HRA group compared to the THA group. In an attempt to quantify the volume of bone removed through each procedure, Su et al. [37] performed a cadaveric study recording the weight of acetabular reamings from either procedure. Whilst no difference was demonstrated once controlled for the native femoral head, it should be noted that the cadavers were non-arthritic, with normal anatomy, which will affect the volume of bone and cartilage resected. Brennan et al. [25] failed to demonstrate any difference between the weight of reamings in a population of non-randomised patients receiving either HRA or THA. We have shown that in our cohort THA was worse at restoring femoral offset and leg length. However, a number of authors have found the opposite [30,38,39]. In certain circumstances, this ability to vary offset and length may be desirable, for example in the case of significant pre-operative shortening or lack of soft tissue tension. The limitations of each design to adjust these variables should be considered during pre-operative planning. It should be noted that the changes in offset and leg lengths in our study were very small, in the order of millimetres, and thus although numerically statistically significant may not necessarily be of any clinical significance. Indeed previous studies have shown that mild radiological changes correlate poorly with clinical outcomes [40] and in particular perception of leg length discrepancy and true leg length inequality correlate poorly [41]. It is thus perhaps unsurprising that although there were radiological differences in our two groups there were no differences in clinical scores. We did not compare this change in leg length with the pre-operative leg length discrepancy but rather looked only at the change in length of the operated leg. Whilst this is a potential limitation of this study in that we have not addressed the issue of restoration of anatomy, rather looked only at a change in anatomy, this does highlight the limitation of HRA to significantly address leg length discrepancy. This finding is consistent with others who have also showed no differences in patient reported outcomes between THA and HRA in both cohort studies [42–45] and randomised controlled trials [15]. Patient reported outcomes tend to be dominated by measures of pain and some
authors have demonstrated better objectively measured pure functional outcomes following HRA in comparison to THA [14,35,46–51]. Whilst our study differs in design to many of those previously presented, and suffers from the same drawbacks as many of these previous studies (non-randomised design, retrospective cohort analysis and nonmatched patient populations), our findings agree with those of others in that despite variations in restoration of hip mechanics and anatomy, this does not translate into a difference in functional or patient reported outcome according to contemporary available outcome measures. Conclusion The amount of bone resected from the acetabulum (as measured by the size of acetabular implant) does not differ between hip resurfacing arthroplasty and total hip arthroplasty. 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