ORTHOPAEDICS ANZJSurg.com
Femoral neck remodelling after hip resurfacing surgery: a radiological study Lawrence Kohan,*† Clarice Field,* Dennis Kerr* and Besim Ben-Nissan† *Joint Orthopaedic Centre, Sydney, New South Wales, Australia and †University of Technology Sydney, Sydney, New South Wales, Australia
Key words femoral neck width, hip resurfacing, neck remodelling. Correspondence Professor Lawrence Kohan, Joint Orthopaedic Centre, PO Box 240, Bondi Junction, NSW 2022, Australia. Email: [email protected] L. Kohan MBBS (Hons), FRACS, FAOrthA, PhD; C. Field BE (Hons), PhD; D. Kerr MBBS, FANZCA; B. Ben-Nissan MSc, PhD. Accepted for publication 10 April 2014. doi: 10.1111/ans.12706
Abstract Background: Narrowing of the femoral neck under the femoral component of the hip resurfacing has been noted previously and has raised concern. In this study we examined the X-rays of patients following Birmingham hip resurfacing surgery at 6-years follow-up. Methods: Bony changes proximally and distally were measured. Fifty-two patients were available for evaluation. Results: There were 40 (76.9%) men and 12 (23.1%) women, with a mean age of 52 years (25–64). The unoperated contralateral femoral neck was measured as a control. We found femoral neck narrowing proximally in 82.7% of patients and distally in 26.9% and on the contralateral side in 54.5%. The average narrowing was 3.6%. Widening was observed proximally in 17.3% and distally in 73.1% and on the contralateral side in 45.5%. The average widening was 3.9%. Four of the 52 patients had proximal narrowing exceeding 10% of the femoral neck diameter, and one of the 52 patients had inferior narrowing exceeding 10%. Conclusion: Gender, body mass index, component size and age did not affect remodelling. We conclude that the observed findings are likely to be a manifestation of a generalized remodelling response in the femoral neck rather than a localized and isolated narrowing at the junction of the component and the femoral neck.
Introduction One of the main advantages of hip resurfacing surgery is the preservation of femoral head and neck bone stock.1 Hip resurfacing may load the femur physiologically thereby reducing bone loss due to stress shielding.1 Dual-energy X-ray absorptiometry (DEXA) analysis of the femoral neck2 found good bone preservation but did not assess the shape of the femoral neck. However, thinning of the femoral neck has been found3 immediately under the femoral component. The long-term effect of this is unclear but there is concern about increased potential for femoral neck fracture. We examined changes in the width of the femoral neck immediately under the femoral component and distally at the base of the femoral neck at 6-years follow-up.
Methods Eighty-eight consecutive resurfacing arthroplasties were performed between January 2003 and December 2003 by the senior author. There were 88 patients: 19 were women (21.6%) and 69 men © 2014 Royal Australasian College of Surgeons
(78.4%). The mean age was 52 years (25–64). Mean body mass index (BMI) was 28.3 (range 20.2–46.2; standard deviation [SD] 4.3). The X-rays of 52 of the 88 resurfacings were assessed at 6-years follow-up. In five patients the X-rays were unsatisfactory, due to femoral neck lack of clarity. Twenty-two patients had poor X-ray imaging or whose images were lost. Twenty-nine patients were not available to have X-rays at the designated follow-up point. Of the 52 patients, there were 40 men (76.9%) and 12 women (23.1%). Mean age was 52 years (25–64). BMI mean was 29.2 (20.5–46.2). Statistically, the study group of 52 was not significantly different from the total group of 88 patients. The size of the femoral and acetabular components ranged from 42 mm to 58 mm and 50 mm to 66 mm respectively. All patients had osteoarthritis. None had previous surgery and there was no history of inflammatory disease. The patients studied had not been treated with steroids, cytotoxic agents, bisphosphonates or radiotherapy. A standard posterior approach was used for all patients. The Birmingham prosthesis (Smith & Nephew, Memphis, TN, USA) was used. The technique used was as described by McMinn.1 Early mobilization protocol with full-weight bearing ANZ J Surg 84 (2014) 639–642
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Fig. 1. Inferior neck width measurement (top left). Superior neck width measurement (top right). Reference anatomical axis (bottom left). Reference neck width measurement (bottom right).
as tolerated4 was used. The component position aimed for a neutral to valgus femoral component alignment within the femoral neck in the coronal plane, and the stem of the femoral component to be positioned as centrally as possible within the femoral neck in the sagittal plane. The acetabular component was positioned with 15° of operative anteversion and 45 to 55° of closure. Radiographs were taken immediately post-operatively and at 6-years follow-up. All radiographs taken in the study were standardized anteroposterior (AP) views of the pelvis centred on the pubic symphysis. The X-ray tube was positioned 1 m above the X-ray plate. The X-rays were performed so that the inferior edge of the femoral component was a straight line, and not oval-shaped, indicating that the plane of the open-end of the component was parallel to the X-ray beam. In order to maintain consistency within the radiographical analysis of the femoral neck width (N) the lesser trochanter vent drill hole (performed during all resurfacing procedures) was set as a datum point of reference. A line was drawn along the axis of the stem of the prosthesis (Fig. 1). A line perpendicular to the previously drawn axial line was drawn from this datum point to intersect the axial line. The line was then moved 10 mm proximally to allow an inferior neck measurement (Fig. 1). The inferior neck width (N) was measured from the axial line to the outer neck cortical bony contour. Only the medial radius was considered because extending the measurement laterally involved the greater trochanter, making the measurement inconsistent. The superior neck width was measured at the distal edge of the femoral component (NS) incorporating the total neck diameter (Fig. 1). To correct for variations in magnification we measured the axial length (IC) of the prosthesis on the radiograph and derived a ratio (R) by diving this measurement by the prosthesis length (IT) on the template (Eqn 1). The template for the prosthesis was 115% of the actual prosthesis size and we corrected this to 100% prosthesis size.
This allowed the creation of a correction factor for any magnification seen on X-ray and allowed us to determine the actual neck (AN) (Eqn 2) and junction (AJN) (Eqn 3) widths from the radiograph. Eqn 1
R=
IC IT
Eqn 2
AN = N × R
Eqn 3
AJN = JN × R
We also looked at the neck width changes on the non-operated side. This reference group included 21 patients: eight were women (38.1%) and 13 were men (61.9%). The average age for the reference group was 49.6 years (range 25–63). This group was not statistically different to the main group of 52 patients. In the reference group the neck width (NR) was measured at the neck–head junction on the non-operated hip and was based on anatomical axis for the femoral neck and using the lesser trochanter as a reference point (Fig. 1). A measurement of the neck width was taken at its narrowest point, perpendicular to this line. Having established this point, its distance from the lesser trochanter point was determined, so that it could be reproduced in subsequent X-rays. The same magnification correction technique was applied as for the operated side. The patient data measurements were performed using Rhinoceros 3D (Robert McNeel & Associates, Seattle, WA, USA). Statistical analysis was performed in SPSSv17 (SPSS Inc., Chicago, IL, USA). Matched pairs t-tests were applied to determine statistical significance in neck width changes. Variable associations were evaluated using bivariate correlations. Linear regression was used to analyse dependencies in neck width changes relative to component size, gender and age. Independent t-tests were applied to assess significant differences in gender, age and BMI. Statistical power was © 2014 Royal Australasian College of Surgeons
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Fig. 3. Percentage neck width in relation to age.
Fig. 2. Superior, inferior and reference percentage neck width changes.
evaluated using G*Power 3.1.0 (Heinrich Heine, Universitat Dusseldorf, Germany) with means difference, post hoc analyses. Measurement consistency was confirmed by repeating the radiological measurements in a random sample of 15. A Pearson’s bivariate correlation coefficient was used to calculate intra-observer reliability (>0.1-mm difference). The difference observed within the test sample was R = 0.602, which was not significant.
Results Figure 2 shows percentage changes in neck width. The average superior neck width change observed was −3.60% (SD −4.17; range −11.33% to +6.33%). The average inferior neck width change was +3.88% (SD −7.89; range −14.64% to +25.31%). On the nonoperated reference side the average neck width change was −0.77% (SD –4.94; range −13.89% to 7.99%). There was narrowing proximally (P < 0.001, power = 0.99) and widening distally (P < 0.001, power = 0.95). The reference group also showed a small but significant change in neck width P < 0.001 (power = 0.98). Narrowing of the femoral neck was observed superiorly (82.7% of patients), inferiorly (26.9%) and in the reference (54.5%) regions. Superiorly, four patients (two women and two men) showed greater than 10% neck narrowing. Inferiorly, one male patient exhibited greater than 10% neck narrowing and same patient on the reference side showed a greater than 10% neck narrowing. Increases in neck width in the superior, inferior and reference sites were in 17.3%, 73.1% and 45.5% patients respectively. All patients showed some changes. The patients were divided into groups, one with a BMI less than 30 and another with a BMI of 30 and above. There were two groups: BMI < 30 and BMI > 30. Narrowing of the proximal aspect under the femoral component, widening at the base of the femoral neck and reference neck width were not significantly related (P > 0.05) to BMI. The influence of age is shown in Figure 3. The percentage changes in the neck diameter were plotted in relation to the age of the patient at the time of the index procedure. Correlation analysis for inferior (R = 0.229), superior (R = 0.320) and reference (R = © 2014 Royal Australasian College of Surgeons
0.522) regions indicated no significant relationship with respect to age of the patient. The linear regression analysis of age, gender, BMI and femoral component sizes found no dependencies associated with superior, inferior and reference neck width changes at 6 years.
Discussion The observation that femoral neck narrowing occurs after hip resurfacing5 is of concern. It carries with it implications of fragility, possibly leading to femoral neck fracture and loss of component support, potentially leading to femoral component loosening. We note that remodelling of the femoral neck occurs with narrowing proximally, just underneath the femoral component and widening distally. Remodelling also occurs on the reference, unoperated side with a slight degree of narrowing of the femoral neck. The femoral neck narrowing just underneath the femoral component is consistent with other reported studies. Hing et al.5 reviewed X-rays of patients following resurfacing surgery using the Birmingham prosthesis found neck narrowing in 77% of their 163 hip resurfacings, and in 27% of patients the narrowing exceeded 10% of the diameter of the femoral neck with a mean follow-up of 5 years. They measured the diameter of the lower border of the femoral component and also made no allowance for variations in femoral version, component seating or cement mantle thickness. The statistical power was not stated. Katrana et al.6 compared the neck narrowing seen in a cemented (Birmingham hip resurfacing) and cementless (Cormet hip resurfacing, Corin, UK) hip resurfacing arthroplasties. They also found narrowing of the femoral neck immediately below the femoral component. Statistical power was not stated. Apart from the proximal narrowing we have also seen widening of the femoral neck at the base. Both of these findings have statistical significance and power. These changes are likely to be a manifestation of bone remodelling in response to altered mechanical environment produced by the femoral component. Previous DEXA analysis2 showed that the quantity of bone in the femoral neck is unchanged and our findings suggest that what we are seeing is a redistribution of bone within the femoral neck. Bone has been resorbed proximally and deposited distally in response to altered stresses. This is consistent with finite element analysis studies.7 We may assume that at 6-years follow-up these changes have stabilized. Hing et al.5 found no change in the femoral neck narrowing beyond 3 years.
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There were limitations to this study. The post-operative X-ray evaluation examined the AP X-rays only. The lesser trochanter vent site was visible in the majority of X-rays. If not, that patient was excluded from the study. We did not look at the sagittal measurements of neck width, and that may affect the results. Further, we did not take into account version of the femoral components, possibility of incomplete seating and cement mantle variability. These factors may affect neck remodelling. On the reference site, a minor average narrowing (0.77%) was noted but the range was quite wide (−13.89% to +7.9%), indicating that a somewhat unpredictable bone response can be expected under normal circumstances without surgical intervention. We did not look at the confounding effect of concurrent medications. While we did exclude patients who had taken corticosteroids and antiepileptic medications because of the adverse effect on bone density, we note that bone density can be adversely affected by medications in common use, such as proton pump inhibitors8 and positively by statins.9 We assume that it is the physical presence of the prosthesis which is responsible for the altered remodelling findings. However, the debris which is produced by the surgery produces mediatorsstimulating osteoclasts.10 This surgical trauma and healing response may have an effect without the necessity of involving the mechanosensor system to explain the remodelling.11 Another possibility could be the development and repair of micro-cracks in the femoral neck. The number and distribution of these may alter with the surgery, and with the presence of the prosthesis. This is a natural process in any bone, and considered essential to bone homoeostasis.12 Following surgery, changes in the bone matrix may alter the mechanical signals received by the osteocytes11 and this may also influence remodelling. Vascular alterations as a result of the surgery have been documented and proposed as being mechanisms to explain the changes seen in the femoral neck.5 Most of the changes described relate to the femoral head and the previous epiphyseal portion, mostly removed at surgery. A good vascular supply to the neck has been shown, intra-osseous in nature, which does not seem to be as vulnerable to surgical trauma. The head–neck junction vascularity appears to be well maintained after resurfacing surgery.13 Extra-osseous causes may also affect the femoral neck shape. The micro-environment around a hip resurfacing is not normal. The fluid has abnormal content and abnormal flow.14 In the normal hip, there is no increase in intracapsular pressure in the normal range of movement. Extra fluid decreases the range in which no extra pressure occurs. Abnormal function in resurfaced hips has been associated with effusions and cysts.15 Fluid pressure and fluid flow are sensed by osteocytes and this may be a stimulus for remodelling.16 In addition to a volume load within the hip joint cavity, there are fluid flow currents which may be responsible for a remodelling stimulus. Cells respond differently to different flow patterns. Fluid flow possibly more than pressure may be effective in generating resorption, or at least osteoclast activation.16 Complex flow eddies are generated adjacent to the articular surface and may result in neck narrowing adjacent to the femoral component edge, which has been observed. Metal toxicity is another possibility. Increased levels of cobalt, chromium and molybdenum ions have been shown to be present. There is an initial ‘wearing in’ phenomenon after surgery with a gradual subsequent decrease in cobalt and chromium ions. Very low
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levels of ions have been described in McKee-Farrar implants in situ for over 20 years.17 However, very high ion levels both locally and systemically18 have also been found. These have been associated with cysts and pseudo-tumours possibly related to toxicity. Patients assessed in this study did not show any of these findings. It is possible, even likely, that many of these factors play a part in the remodelling, producing the appearance seen on X-ray.
References 1. McMinn DJW. Modern Hip Resurfacing. London, UK: Springer, 2010. 2. Cordingley R, Kohan L, Ben-Nissan B. What happens to femoral neck bone mineral density after hip resurfacing surgery? J. Bone Joint Surg. Br. 2010; 92: 1648–53. 3. Spencer S, Carter R, Murray H, Meek RM. Femoral neck narrowing after metal-on-metal hip resurfacing. J. Arthroplasty 2008; 23: 1105–9. 4. Kerr DR, Kohan L. Local infiltration analgesia: a technique for the control of acute postoperative pain following knee and hip surgery: a case study of 325 patients. Acta Orthop. 2008; 79: 174–83. 5. Hing CB, Young DA, Dalziel RE, Bailey M, Back DL, Shimmin AJ. Narrowing of the neck in resurfacing arthroplasty of the hip: a radiological study. J. Bone Joint Surg. Br. 2007; 89: 1019–24. 6. Katrana P, Crawford J, Vowler S, Lilikakis A, Villar R. Femoral neck resorption after hip resurfacing arthroplasty – a comparison of cemented and uncemented prostheses. J. Bone Joint Surg. Br. 2006; 88-B (SUPP_II): 234-c-. 7. Kohan L. Surgical Aspects, Finite Element Analysis and X-ray Correlation of Femoral Neck Changes in the Osteoarthritic Hip after Hip Resurfacing Surgery. Sydney: University of Technology Sydney, 2010. 8. Vestergaard P, Rejnmark L, Mosekilde L. Proton pump inhibitors, histamine H2 receptor antagonists, and other antacid medications and the risk of fracture. Calcif. Tissue Int. 2006; 79: 76–83. 9. Pasco JA, Kotowicz MA, Henry MJ, Sanders KM, Nicholson GC. Statin use, bone mineral density, and fracture risk: Geelong Osteoporosis Study. Arch. Intern. Med. 2002; 162: 537–40. 10. Cardoso L, Herman BC, Verborgt O, Laudier D, Majeska RJ, Schaffler MB. Osteocyte apoptosis controls activation of intracortical resorption in response to bone fatigue. J. Bone Miner. Res. 2009; 24: 597–605. 11. Potter R, Havill L, Nicollella D (eds). Microcrack Bone Tissue Strains Around the Osteocyte Lacuna in Young and Old Bone. New Orleans, USA: Orthopaedic Research Society, 2010. 12. Burr DB. Targeted and nontargeted remodeling. Bone 2002; 30: 2–4. 13. McMahon SJ, Young D, Ballok Z, Badaruddin BS, Larbpaiboonpong V, Hawdon G. Vascularity of the femoral head after Birmingham hip resurfacing. A technetium Tc 99 m bone scan/single photon emission computed tomography study. J. Arthroplasty 2006; 21: 514–21. 14. Konttinen YT, Zhao D, Beklen A et al. The microenvironment around total hip replacement prostheses. Clin. Orthop. Relat. Res. 2005; 430: 28–38. 15. Pandit H, Glyn-Jones S, McLardy-Smith P et al. Pseudotumours associated with metal-on-metal hip resurfacings. J. Bone Joint Surg. Br. 2008; 90: 847–51. 16. Qin YX, Kaplan T, Saldanha A, Rubin C. Fluid pressure gradients, arising from oscillations in intramedullary pressure, is correlated with the formation of bone and inhibition of intracortical porosity. J. Biomech. 2003; 36: 1427–37. 17. Jacobs JJ, Skipor AK, Doorn PF et al. Cobalt and chromium concentrations in patients with metal on metal total hip replacements. Clin. Orthop. Relat. Res. 1996; 329 (Suppl.): S256–63. 18. Haddad FS, Thakrar RR, Hart AJ et al. Metal-on-metal bearings: the evidence so far. J. Bone Joint Surg. Br. 2011; 93: 572–9.
© 2014 Royal Australasian College of Surgeons
Femoral neck remodelling after hip resurfacing surgery: a radiological study Lawrence Kohan,*† Clarice Field,* Dennis Kerr* and Besim Ben-Nissan† *Joint Orthopaedic Centre, Sydney, New South Wales, Australia and †University of Technology Sydney, Sydney, New South Wales, Australia
Key words femoral neck width, hip resurfacing, neck remodelling. Correspondence Professor Lawrence Kohan, Joint Orthopaedic Centre, PO Box 240, Bondi Junction, NSW 2022, Australia. Email: [email protected] L. Kohan MBBS (Hons), FRACS, FAOrthA, PhD; C. Field BE (Hons), PhD; D. Kerr MBBS, FANZCA; B. Ben-Nissan MSc, PhD. Accepted for publication 10 April 2014. doi: 10.1111/ans.12706
Abstract Background: Narrowing of the femoral neck under the femoral component of the hip resurfacing has been noted previously and has raised concern. In this study we examined the X-rays of patients following Birmingham hip resurfacing surgery at 6-years follow-up. Methods: Bony changes proximally and distally were measured. Fifty-two patients were available for evaluation. Results: There were 40 (76.9%) men and 12 (23.1%) women, with a mean age of 52 years (25–64). The unoperated contralateral femoral neck was measured as a control. We found femoral neck narrowing proximally in 82.7% of patients and distally in 26.9% and on the contralateral side in 54.5%. The average narrowing was 3.6%. Widening was observed proximally in 17.3% and distally in 73.1% and on the contralateral side in 45.5%. The average widening was 3.9%. Four of the 52 patients had proximal narrowing exceeding 10% of the femoral neck diameter, and one of the 52 patients had inferior narrowing exceeding 10%. Conclusion: Gender, body mass index, component size and age did not affect remodelling. We conclude that the observed findings are likely to be a manifestation of a generalized remodelling response in the femoral neck rather than a localized and isolated narrowing at the junction of the component and the femoral neck.
Introduction One of the main advantages of hip resurfacing surgery is the preservation of femoral head and neck bone stock.1 Hip resurfacing may load the femur physiologically thereby reducing bone loss due to stress shielding.1 Dual-energy X-ray absorptiometry (DEXA) analysis of the femoral neck2 found good bone preservation but did not assess the shape of the femoral neck. However, thinning of the femoral neck has been found3 immediately under the femoral component. The long-term effect of this is unclear but there is concern about increased potential for femoral neck fracture. We examined changes in the width of the femoral neck immediately under the femoral component and distally at the base of the femoral neck at 6-years follow-up.
Methods Eighty-eight consecutive resurfacing arthroplasties were performed between January 2003 and December 2003 by the senior author. There were 88 patients: 19 were women (21.6%) and 69 men © 2014 Royal Australasian College of Surgeons
(78.4%). The mean age was 52 years (25–64). Mean body mass index (BMI) was 28.3 (range 20.2–46.2; standard deviation [SD] 4.3). The X-rays of 52 of the 88 resurfacings were assessed at 6-years follow-up. In five patients the X-rays were unsatisfactory, due to femoral neck lack of clarity. Twenty-two patients had poor X-ray imaging or whose images were lost. Twenty-nine patients were not available to have X-rays at the designated follow-up point. Of the 52 patients, there were 40 men (76.9%) and 12 women (23.1%). Mean age was 52 years (25–64). BMI mean was 29.2 (20.5–46.2). Statistically, the study group of 52 was not significantly different from the total group of 88 patients. The size of the femoral and acetabular components ranged from 42 mm to 58 mm and 50 mm to 66 mm respectively. All patients had osteoarthritis. None had previous surgery and there was no history of inflammatory disease. The patients studied had not been treated with steroids, cytotoxic agents, bisphosphonates or radiotherapy. A standard posterior approach was used for all patients. The Birmingham prosthesis (Smith & Nephew, Memphis, TN, USA) was used. The technique used was as described by McMinn.1 Early mobilization protocol with full-weight bearing ANZ J Surg 84 (2014) 639–642
640
Kohan et al.
Fig. 1. Inferior neck width measurement (top left). Superior neck width measurement (top right). Reference anatomical axis (bottom left). Reference neck width measurement (bottom right).
as tolerated4 was used. The component position aimed for a neutral to valgus femoral component alignment within the femoral neck in the coronal plane, and the stem of the femoral component to be positioned as centrally as possible within the femoral neck in the sagittal plane. The acetabular component was positioned with 15° of operative anteversion and 45 to 55° of closure. Radiographs were taken immediately post-operatively and at 6-years follow-up. All radiographs taken in the study were standardized anteroposterior (AP) views of the pelvis centred on the pubic symphysis. The X-ray tube was positioned 1 m above the X-ray plate. The X-rays were performed so that the inferior edge of the femoral component was a straight line, and not oval-shaped, indicating that the plane of the open-end of the component was parallel to the X-ray beam. In order to maintain consistency within the radiographical analysis of the femoral neck width (N) the lesser trochanter vent drill hole (performed during all resurfacing procedures) was set as a datum point of reference. A line was drawn along the axis of the stem of the prosthesis (Fig. 1). A line perpendicular to the previously drawn axial line was drawn from this datum point to intersect the axial line. The line was then moved 10 mm proximally to allow an inferior neck measurement (Fig. 1). The inferior neck width (N) was measured from the axial line to the outer neck cortical bony contour. Only the medial radius was considered because extending the measurement laterally involved the greater trochanter, making the measurement inconsistent. The superior neck width was measured at the distal edge of the femoral component (NS) incorporating the total neck diameter (Fig. 1). To correct for variations in magnification we measured the axial length (IC) of the prosthesis on the radiograph and derived a ratio (R) by diving this measurement by the prosthesis length (IT) on the template (Eqn 1). The template for the prosthesis was 115% of the actual prosthesis size and we corrected this to 100% prosthesis size.
This allowed the creation of a correction factor for any magnification seen on X-ray and allowed us to determine the actual neck (AN) (Eqn 2) and junction (AJN) (Eqn 3) widths from the radiograph. Eqn 1
R=
IC IT
Eqn 2
AN = N × R
Eqn 3
AJN = JN × R
We also looked at the neck width changes on the non-operated side. This reference group included 21 patients: eight were women (38.1%) and 13 were men (61.9%). The average age for the reference group was 49.6 years (range 25–63). This group was not statistically different to the main group of 52 patients. In the reference group the neck width (NR) was measured at the neck–head junction on the non-operated hip and was based on anatomical axis for the femoral neck and using the lesser trochanter as a reference point (Fig. 1). A measurement of the neck width was taken at its narrowest point, perpendicular to this line. Having established this point, its distance from the lesser trochanter point was determined, so that it could be reproduced in subsequent X-rays. The same magnification correction technique was applied as for the operated side. The patient data measurements were performed using Rhinoceros 3D (Robert McNeel & Associates, Seattle, WA, USA). Statistical analysis was performed in SPSSv17 (SPSS Inc., Chicago, IL, USA). Matched pairs t-tests were applied to determine statistical significance in neck width changes. Variable associations were evaluated using bivariate correlations. Linear regression was used to analyse dependencies in neck width changes relative to component size, gender and age. Independent t-tests were applied to assess significant differences in gender, age and BMI. Statistical power was © 2014 Royal Australasian College of Surgeons
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Fig. 3. Percentage neck width in relation to age.
Fig. 2. Superior, inferior and reference percentage neck width changes.
evaluated using G*Power 3.1.0 (Heinrich Heine, Universitat Dusseldorf, Germany) with means difference, post hoc analyses. Measurement consistency was confirmed by repeating the radiological measurements in a random sample of 15. A Pearson’s bivariate correlation coefficient was used to calculate intra-observer reliability (>0.1-mm difference). The difference observed within the test sample was R = 0.602, which was not significant.
Results Figure 2 shows percentage changes in neck width. The average superior neck width change observed was −3.60% (SD −4.17; range −11.33% to +6.33%). The average inferior neck width change was +3.88% (SD −7.89; range −14.64% to +25.31%). On the nonoperated reference side the average neck width change was −0.77% (SD –4.94; range −13.89% to 7.99%). There was narrowing proximally (P < 0.001, power = 0.99) and widening distally (P < 0.001, power = 0.95). The reference group also showed a small but significant change in neck width P < 0.001 (power = 0.98). Narrowing of the femoral neck was observed superiorly (82.7% of patients), inferiorly (26.9%) and in the reference (54.5%) regions. Superiorly, four patients (two women and two men) showed greater than 10% neck narrowing. Inferiorly, one male patient exhibited greater than 10% neck narrowing and same patient on the reference side showed a greater than 10% neck narrowing. Increases in neck width in the superior, inferior and reference sites were in 17.3%, 73.1% and 45.5% patients respectively. All patients showed some changes. The patients were divided into groups, one with a BMI less than 30 and another with a BMI of 30 and above. There were two groups: BMI < 30 and BMI > 30. Narrowing of the proximal aspect under the femoral component, widening at the base of the femoral neck and reference neck width were not significantly related (P > 0.05) to BMI. The influence of age is shown in Figure 3. The percentage changes in the neck diameter were plotted in relation to the age of the patient at the time of the index procedure. Correlation analysis for inferior (R = 0.229), superior (R = 0.320) and reference (R = © 2014 Royal Australasian College of Surgeons
0.522) regions indicated no significant relationship with respect to age of the patient. The linear regression analysis of age, gender, BMI and femoral component sizes found no dependencies associated with superior, inferior and reference neck width changes at 6 years.
Discussion The observation that femoral neck narrowing occurs after hip resurfacing5 is of concern. It carries with it implications of fragility, possibly leading to femoral neck fracture and loss of component support, potentially leading to femoral component loosening. We note that remodelling of the femoral neck occurs with narrowing proximally, just underneath the femoral component and widening distally. Remodelling also occurs on the reference, unoperated side with a slight degree of narrowing of the femoral neck. The femoral neck narrowing just underneath the femoral component is consistent with other reported studies. Hing et al.5 reviewed X-rays of patients following resurfacing surgery using the Birmingham prosthesis found neck narrowing in 77% of their 163 hip resurfacings, and in 27% of patients the narrowing exceeded 10% of the diameter of the femoral neck with a mean follow-up of 5 years. They measured the diameter of the lower border of the femoral component and also made no allowance for variations in femoral version, component seating or cement mantle thickness. The statistical power was not stated. Katrana et al.6 compared the neck narrowing seen in a cemented (Birmingham hip resurfacing) and cementless (Cormet hip resurfacing, Corin, UK) hip resurfacing arthroplasties. They also found narrowing of the femoral neck immediately below the femoral component. Statistical power was not stated. Apart from the proximal narrowing we have also seen widening of the femoral neck at the base. Both of these findings have statistical significance and power. These changes are likely to be a manifestation of bone remodelling in response to altered mechanical environment produced by the femoral component. Previous DEXA analysis2 showed that the quantity of bone in the femoral neck is unchanged and our findings suggest that what we are seeing is a redistribution of bone within the femoral neck. Bone has been resorbed proximally and deposited distally in response to altered stresses. This is consistent with finite element analysis studies.7 We may assume that at 6-years follow-up these changes have stabilized. Hing et al.5 found no change in the femoral neck narrowing beyond 3 years.
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There were limitations to this study. The post-operative X-ray evaluation examined the AP X-rays only. The lesser trochanter vent site was visible in the majority of X-rays. If not, that patient was excluded from the study. We did not look at the sagittal measurements of neck width, and that may affect the results. Further, we did not take into account version of the femoral components, possibility of incomplete seating and cement mantle variability. These factors may affect neck remodelling. On the reference site, a minor average narrowing (0.77%) was noted but the range was quite wide (−13.89% to +7.9%), indicating that a somewhat unpredictable bone response can be expected under normal circumstances without surgical intervention. We did not look at the confounding effect of concurrent medications. While we did exclude patients who had taken corticosteroids and antiepileptic medications because of the adverse effect on bone density, we note that bone density can be adversely affected by medications in common use, such as proton pump inhibitors8 and positively by statins.9 We assume that it is the physical presence of the prosthesis which is responsible for the altered remodelling findings. However, the debris which is produced by the surgery produces mediatorsstimulating osteoclasts.10 This surgical trauma and healing response may have an effect without the necessity of involving the mechanosensor system to explain the remodelling.11 Another possibility could be the development and repair of micro-cracks in the femoral neck. The number and distribution of these may alter with the surgery, and with the presence of the prosthesis. This is a natural process in any bone, and considered essential to bone homoeostasis.12 Following surgery, changes in the bone matrix may alter the mechanical signals received by the osteocytes11 and this may also influence remodelling. Vascular alterations as a result of the surgery have been documented and proposed as being mechanisms to explain the changes seen in the femoral neck.5 Most of the changes described relate to the femoral head and the previous epiphyseal portion, mostly removed at surgery. A good vascular supply to the neck has been shown, intra-osseous in nature, which does not seem to be as vulnerable to surgical trauma. The head–neck junction vascularity appears to be well maintained after resurfacing surgery.13 Extra-osseous causes may also affect the femoral neck shape. The micro-environment around a hip resurfacing is not normal. The fluid has abnormal content and abnormal flow.14 In the normal hip, there is no increase in intracapsular pressure in the normal range of movement. Extra fluid decreases the range in which no extra pressure occurs. Abnormal function in resurfaced hips has been associated with effusions and cysts.15 Fluid pressure and fluid flow are sensed by osteocytes and this may be a stimulus for remodelling.16 In addition to a volume load within the hip joint cavity, there are fluid flow currents which may be responsible for a remodelling stimulus. Cells respond differently to different flow patterns. Fluid flow possibly more than pressure may be effective in generating resorption, or at least osteoclast activation.16 Complex flow eddies are generated adjacent to the articular surface and may result in neck narrowing adjacent to the femoral component edge, which has been observed. Metal toxicity is another possibility. Increased levels of cobalt, chromium and molybdenum ions have been shown to be present. There is an initial ‘wearing in’ phenomenon after surgery with a gradual subsequent decrease in cobalt and chromium ions. Very low
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levels of ions have been described in McKee-Farrar implants in situ for over 20 years.17 However, very high ion levels both locally and systemically18 have also been found. These have been associated with cysts and pseudo-tumours possibly related to toxicity. Patients assessed in this study did not show any of these findings. It is possible, even likely, that many of these factors play a part in the remodelling, producing the appearance seen on X-ray.
References 1. McMinn DJW. Modern Hip Resurfacing. London, UK: Springer, 2010. 2. Cordingley R, Kohan L, Ben-Nissan B. What happens to femoral neck bone mineral density after hip resurfacing surgery? J. Bone Joint Surg. Br. 2010; 92: 1648–53. 3. Spencer S, Carter R, Murray H, Meek RM. Femoral neck narrowing after metal-on-metal hip resurfacing. J. Arthroplasty 2008; 23: 1105–9. 4. Kerr DR, Kohan L. Local infiltration analgesia: a technique for the control of acute postoperative pain following knee and hip surgery: a case study of 325 patients. Acta Orthop. 2008; 79: 174–83. 5. Hing CB, Young DA, Dalziel RE, Bailey M, Back DL, Shimmin AJ. Narrowing of the neck in resurfacing arthroplasty of the hip: a radiological study. J. Bone Joint Surg. Br. 2007; 89: 1019–24. 6. Katrana P, Crawford J, Vowler S, Lilikakis A, Villar R. Femoral neck resorption after hip resurfacing arthroplasty – a comparison of cemented and uncemented prostheses. J. Bone Joint Surg. Br. 2006; 88-B (SUPP_II): 234-c-. 7. Kohan L. Surgical Aspects, Finite Element Analysis and X-ray Correlation of Femoral Neck Changes in the Osteoarthritic Hip after Hip Resurfacing Surgery. Sydney: University of Technology Sydney, 2010. 8. Vestergaard P, Rejnmark L, Mosekilde L. Proton pump inhibitors, histamine H2 receptor antagonists, and other antacid medications and the risk of fracture. Calcif. Tissue Int. 2006; 79: 76–83. 9. Pasco JA, Kotowicz MA, Henry MJ, Sanders KM, Nicholson GC. Statin use, bone mineral density, and fracture risk: Geelong Osteoporosis Study. Arch. Intern. Med. 2002; 162: 537–40. 10. Cardoso L, Herman BC, Verborgt O, Laudier D, Majeska RJ, Schaffler MB. Osteocyte apoptosis controls activation of intracortical resorption in response to bone fatigue. J. Bone Miner. Res. 2009; 24: 597–605. 11. Potter R, Havill L, Nicollella D (eds). Microcrack Bone Tissue Strains Around the Osteocyte Lacuna in Young and Old Bone. New Orleans, USA: Orthopaedic Research Society, 2010. 12. Burr DB. Targeted and nontargeted remodeling. Bone 2002; 30: 2–4. 13. McMahon SJ, Young D, Ballok Z, Badaruddin BS, Larbpaiboonpong V, Hawdon G. Vascularity of the femoral head after Birmingham hip resurfacing. A technetium Tc 99 m bone scan/single photon emission computed tomography study. J. Arthroplasty 2006; 21: 514–21. 14. Konttinen YT, Zhao D, Beklen A et al. The microenvironment around total hip replacement prostheses. Clin. Orthop. Relat. Res. 2005; 430: 28–38. 15. Pandit H, Glyn-Jones S, McLardy-Smith P et al. Pseudotumours associated with metal-on-metal hip resurfacings. J. Bone Joint Surg. Br. 2008; 90: 847–51. 16. Qin YX, Kaplan T, Saldanha A, Rubin C. Fluid pressure gradients, arising from oscillations in intramedullary pressure, is correlated with the formation of bone and inhibition of intracortical porosity. J. Biomech. 2003; 36: 1427–37. 17. Jacobs JJ, Skipor AK, Doorn PF et al. Cobalt and chromium concentrations in patients with metal on metal total hip replacements. Clin. Orthop. Relat. Res. 1996; 329 (Suppl.): S256–63. 18. Haddad FS, Thakrar RR, Hart AJ et al. Metal-on-metal bearings: the evidence so far. J. Bone Joint Surg. Br. 2011; 93: 572–9.
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