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Cup Size and Risk of Dislocation After Primary Total Hip Arthroplasty
The Journal of Arthroplasty Vol. 26 No. 8 2011
Cup Size and Risk of Dislocation After Primary Total Hip Arthroplasty Robin Peter, MD, Anne Lübbeke, MD, DSc, Richard Stern, MD, and Pierre Hoffmeyer, MD
Abstract: We evaluated the relationship between cup diameter and dislocation risk in patients undergoing primary total hip arthroplasty (THA) with a 28-mm head. There were 50 dislocations, 28 of which occurred in 2221 (1.3%) THAs with a cup diameter smaller than 56 mm and 22 in 513 (4.3%) with a cup diameter of 56 mm or larger. Dislocation risk varied between 0.6% and 2.4% in the smaller cup group and between 4.1% and 5.2% in the larger cup group. The risk was substantially higher in the large cup group (unadjusted odds ratio, 3.5; 95% confidence interval, 2.0-6.2). Multivariable logistic regression revealed an adjusted odds ratio of 2.4 (95% confidence interval, 1.2-4.9). Patients with THA (28-mm head) had more than twice the risk of dislocation with cup size of 56 mm or higher compared to patients with smaller cups. Keywords: total hip dislocation, hip arthroplasty dislocation, acetabular cup size: acetabular cup diameter, total hip head size. © 2011 Elsevier Inc. All rights reserved.
One of the most frequent complications after total hip arthroplasty (THA) is dislocation. Although much effort has been devoted to its prevention, the risk of dislocation after primary THA still remains between 2% and 5% [1-3]. Much has been written concerning the etiology and prevention of dislocation. Circumstances such as the patient's overall condition generally cannot be influenced by the surgeon, and obese patients have been reported to be particularly at risk for dislocation [4,5]. Moreover, it has been demonstrated that older age and higher American Society of Anesthesiologists (ASA) scores are aggravating factors [1,6-9]. The relationship between femoral head size and risk of dislocation has been extensively studied, and the general consensus is that the risk of dislocation is greater with smaller head sizes [1,6,10-12]. In addition, other such technical points as surgical approach and correct positioning and orientation of both the cup and femoral implant are known risk factors for dislocation [8,10,11,13-18]. To the best of our knowledge, only 1
From the Division of Orthopaedics and Trauma Surgery, University Hospitals of Geneva, Geneva, Switzerland. Submitted February 12, 2010; accepted November 27, 2010. The conflict-of-interest statement associated with this article can be found at doi:10.1016/j.arth.2010.11.015. Reprint requests: Richard Stern, MD, Division of Orthopaedics and Trauma Surgery, University Hospitals of Geneva, 4, rue Gabrielle Perret-Gentil, 1211 Geneva, Switzerland. © 2011 Elsevier Inc. All rights reserved. 0883-5403/2608-0030$36.00/0 doi:10.1016/j.arth.2010.11.015
study has specifically addressed the relationship between a larger outer cup diameter and an increased risk of dislocation [12]. Our objective was to evaluate the relationship between cup size and risk of dislocation within a large cohort of patients with the same type of implant, surgical approach, and femoral head size, while taking into account other known risk factors.
Material and Methods Study Population and Study Design Since March 1996, all patients undergoing THA in a university hospital are routinely enrolled in a prospective hospital-based registry and followed longitudinally. For this prospective cohort study, we included all primary hybrid (uncemented cup and cemented stem) THAs performed between March 1996 and July 2008. All patients were operated upon in the lateral decubitus position and the surgical incision was directly lateral. The hip joint was exposed via a standard transgluteal approach, splitting the distal part of the gluteus medius as well as the gluteus minimus off the anterior aspect of the greater trochanter. All patients received an uncemented press-fit titanium mesh backed cup (Morscher cup; Zimmer, Ltd, Winterthur, Switzerland) and a cemented stem (Muller straight stem or Virtec stem, Zimmer) with a 28-mm head. Of 2759 THAs eligible for inclusion, 25 (0.9%) had missing information on cup diameter and were excluded, leaving 2341 patients undergoing 2734 THAs. Ethics committee approval was obtained for this study.
1305
1306 The Journal of Arthroplasty Vol. 26 No. 8 December 2011 Study Variables The exposure of interest was the cup diameter analyzed as a categorical variable (ranging from 44 to 64 mm) and as a dichotomized variable (b56 vs ≥56 mm). The main outcome of interest was the occurrence of dislocation within the first year postoperative. A fall from any level of energy resulting in a dislocation was excluded. This was the case for 5 patients. The following covariates (potential confounding factors) of the effect of cup diameter on the risk of dislocation were measured: (a) age at operation, as continuous and dichotomized variable (b70 vs ≥70 years); (b) sex; (c) weight; (d) height; (e) body mass index (BMI) as continuous and dichotomized variable (b30 vs ≥30 kg/m2); (f) American Society of Anesthesiologists (ASA) score investigated as a binary variable (ASA 1-2 vs 3-4); (g) diagnosis (primary or secondary osteoarthritis vs posttraumatic osteoarthritis); (h) surgeon's experience (junior vs senior surgeon); (i) bearing surface (ceramic-polyethylene vs metal-metal); and (j) cup inclination investigated as a continuous and dichotomized variable (≤45° vs N45°). Data Collection Information about the preoperative status and the surgical intervention was routinely documented by the operating surgeon on specifically designed data collection forms. Any dislocation occurring within the first year after surgery and treated at our hospital, or reported to the orthopaedic surgeon during one of the follow-up visits, was systematically included in the database by a trained data manager. The data entry of any complication (including dislocation) or reoperation in a patient with a hip prosthesis was double checked by the physician in charge of the hip registry (A.L.). Furthermore, we periodically verified any main complication related to THA (including dislocation) by comparing the hospital diagnosis coding system and our medical records. Cup inclination was available for patients with dislocation and for a randomly selected subgroup without the event of interest (overall available for n = 337 THAs). It was measured on postoperative anteroposterior radiographs by a physician blinded to the outcome of interest. Statistical Analysis First, the distribution of baseline characteristics was compared among the 2 exposure groups (cup diameter b56 vs ≥56 mm). To estimate the magnitude of the differences between them, we calculated relative risks and 95% confidence intervals (CI) for dichotomized and mean differences and 95% CIs for continuous variables. Second, to assess the association between cup diameter and risk of dislocation, we performed univariable logistic regression. Then, multivariable logistic regression modeling was used to adjust the effect of cup
diameter on the dislocation risk for confounding variables (covariates). Of all the above-mentioned covariates, only age, BMI, and posttraumatic THA turned out to be the confounding factors in the relationship of interest (P b .05). For the final model, we used the dichotomized version of age and BMI. Although the covariate, sex, was not statistically significant, it was maintained in the final model. Furthermore, height was also maintained because adding it to the model changed the point estimate for the relationship of interest by 10% to 15%. This was not the case for the other covariates such as ASA score, bearing surface, cup inclination, and surgeon experience, which were excluded. Third, the relationship between cup diameter and risk of dislocation taking into account other known risk factors such as older age and obesity was further explored by using stratification. Based on the stratified results, a graph was created illustrating the risk of dislocation in the 2 diameter groups in relation to the other risk factors. Data were missing for BMI and/or ASA score for 47 patients (1.7%). Given the low frequency, no imputations were performed. The final regression analysis was done including 2687 THAs (98.3%).
Results In total, 2734 THAs were included (1215 in men, 1519 in women) in 2341 patients with a mean age of 70.6 years (SD, ±10.2) and a mean BMI of 27.0 kg/m2 (SD, ±4.6). In 2221 THAs (81.2%), a cup with a diameter smaller than 56 mm was used, whereas in 513 THAs (18.8%) a cup with a diameter of 56 mm or higher was used. Larger cups were used almost exclusively in men (women, 3.2% of THAs; men, 38.2% of THAs; Table 1). Patients with larger cup diameters were slightly younger, significantly taller, had a higher mean BMI, and, more often, had an ASA score of 3 to 4. In addition, larger cups were more often used by senior surgeons. A total of 50 dislocations occurred within the first year postoperative, 28 (1.3%) in the 2221 THAs with a diameter smaller than 56 mm and 22 (4.3%) in the 513 THAs with a diameter of 56 mm or higher. The dislocation risk in the smaller diameter group varied between 0.6% and 2.4%, whereas the risk in the larger diameter group varied between 4.1% and 5.2% (Fig. 1). There was a substantially higher risk of dislocation in the group with a cup diameter of 56 mm or higher, with an absolute risk increase of 3% and a relative risk increase of 331%. The unadjusted odds ratio was 3.5 (95% CI 2.0-6.2). Multivariable logistic regression (Table 2) adjusting for age, sex, BMI, height, and diagnosis revealed an adjusted odds ratio of 2.4 (95% CI 1.2-4.9). The risk of dislocation in relation to the other risk factors is illustrated in Fig. 2. Compared to the smaller diameter group, the risk was always higher in the group
Cup size and dislocation risk after primary total hip arthroplasty Peter et al
1307
Table 1. Baseline Characteristics According to the 2 Groups (Cup Diameter b56 and ≥56 mm) Cup Diameter b56 mm (n = 2221 hips) Women Men Age, mean (±SD) BMI, mean (±SD)* Height (cm), mean (±SD)* ASA score † 1-2 3-4 Posttraumatic No Yes Surgeon experience Junior Senior Bearing surface Ceramic-polyethylene Metal-metal Cup inclination, ‡ mean (±SD) Cup inclination, N45° ‡
%
Cup Diameter ≥56 mm (n = 513 hips)
%
Relative Risks (95% CI); Mean Difference (95% CI)
P Value
11.8 (8.9 to 15.7) 1.2 (0.2 to 2.1) −1.0 (−0.6 to −1.5) −10.7 (−9.9 to −11.4)
b.001 .022 b.001 b.001
1470 751 70.9 (±10.2) 26.8 (±4.6) 164.0 (±8.0)
96.8 61.8
49 464 69.7 (±10.0) 27.8 (±4.5) 174.7 (±7.4)
3.2 38.2
1626 565
82.6 77.4
342 165
17.4 22.6
1.3 (1.1 to 1.5)
.002
2152 69
81.3 79.3
495 18
18.7 20.7
1.1 (0.7 to 1.7)
.640
1884 337
82.7 74.1
395 118
17.3 25.9
1.5 (1.3 to 1.8)
b.001
1759 462 38.8° (±6.8) 39°
81.4 80.8
403 110 38.6° (±7.5) 16°
18.6 19.2
1.0 (0.9 to 1.2) 0.2 (−1.6 to 1.9) 1.4 (0.8 to 2.3)
.748 .850 .229
15.0
20.8
* Height/BMI missing for 47 THAs. † ASA score missing for 37 THAs. ‡ Cup inclination available for n = 337 THAs (260 in smaller and 77 in larger cup group).
with the larger cup diameters whether there were additional risk factors present or not. The use of a large cup in the presence of older age and obesity was associated with the highest risk of dislocation of 7.5%, as compared to the lowest risk of 0.6% in the younger, nonobese patients fitted with a smaller cup.
Discussion We observed a significant increase in the risk of dislocation within the first year postoperative in primary THAs using a large cup size and a 28-mm head. Comparing smaller (b56 mm) to larger cup sizes (≥56
mm), we found the risk of dislocation to be more than twice as high in the larger size group (4.3% vs 1.3%). This association remained significant even after taking into account other known risk factors. To the best of our knowledge, only 1 study by Kelley et al [12] has thus far addressed the relationship between THA stability and outer acetabular diameter. Using posterior or lateral approaches, these authors compared the rate of dislocation with both 22-mm and 28-mm head sizes and various cup diameters. In a small sample group of only 30 patients (31 THAs), these authors evaluated 2 different head sizes (22 and
Fig. 1. Risk of dislocation according to cup size.
1308 The Journal of Arthroplasty Vol. 26 No. 8 December 2011 Table 2. Effect of Cup Diameter on Dislocation Adjusted for Relevant Confounding Factors Using Multivariate Logistic Regression Odds Ratio (95% CI) Cup diameter b56 vs ≥56 mm (≥56 = 1) Age b70 vs ≥70 y (≥70 = 1) Sex (female = 1) BMI b30 vs ≥30 kg/m2 (obese = 1) Height (continuous) THA Postfracture (yes = 1)
2.4 (1.2 to 4.9) 2.5 (1.3 to 4.7) 0.8 (0.3 to 1.8) 2.1 (1.2 to 3.9) 1.03 (0.98 to 1.07) 4.7 (1.7 to 12.6)
28 mm) with cup sizes of 54 mm or smaller and 56 mm or larger. With such small subgroups, also including 2 different surgical approaches, it is difficult to extract any meaningful conclusions. However, in a second larger study population (n = 308) utilizing only the posterior approach and including only 28-mm head sizes, the use of an outer diameter of 60 mm or larger was associated with a significantly increased risk of dislocation as compared to an outer diameter smaller than 60 mm (11.3% vs 3.9%, respectively). In contrast to the work of Kelley et al [12], our large sample size allowed us to adjust for other known risk factors. In addition to cup diameter, older age (≥70 years), obesity, and postfracture THA remained as statistically significant risk factors in our final model. Previous studies have already shown a significant increase in the risk of dislocation in association with these factors [4,6,7,9,14,19,20]. However, the risk of dislocation was not influenced by the type of bearing surface, sex, ASA score, surgeon's experience, or angle of cup inclination, the other possible confounding factors evaluated in this study. The relationship between sex and cup diameter merits further discussion. The vast majority of THAs in the larger cup diameter group (464/513, 90%) were implanted in men. This led to a crude higher risk of dislocation in men (data not shown) and was related to the disproportionally greater use of larger cup dia-
Fig. 2. Risk of dislocation stratified by the 2 cup-size groups and by age and obesity.
meters among them as seen in the results of the multivariable regression analysis. After adjusting for cup diameter (and the other covariates remaining in the model), the association between sex and dislocation was no longer significant. Kelley et al [12] showed that a large difference between the size of the femoral head and outer cup diameter (mismatch) further increased the risk of dislocation more than simply cup diameter. They proposed 3 reasons for this, including anatomic mismatch, pseudocapsule attachment, and prosthetic impingement. But they provided no evidence for their hypotheses. As regards the pseudocapsule, it is circumferentially attached to the acetabular rim, but never actually adheres to the polyethylene surrounding the cup bore. With a larger acetabular component the pelvic insertion of this pseudocapsule is at a greater distance from the head, which we assume would allow for a larger joint cavity to accommodate the head in the event of a dislocation. On the other hand, a closer match between head and cup outer diameters would restrict the inner size of the neo-capsule and thus reduce the risk of dislocation. In our search of the literature, we found no biomechanical studies that specifically looked at the effect of cup size on hip stability. There are studies using finite element analyses and computer-aided programs to evaluate certain variables as they relate to hip stability and dislocation [11,21-25], but none of these address the issue of cup size. Strengths and Limitations Our first main strength is that our study was conducted prospectively in a large cohort of patients from a university hospital. Secondly, we restricted inclusion to patients operated upon with use of the same surgical approach, the same head size (28 mm), and the same cup. No hooded liners were used. Thirdly, the large sample size of our study population allowed us to adjust for other main known risk factors in addition to head size and surgical approach. One possible limitation of our study concerns loss of patients in follow-up. However, our university hospital is the only public hospital in the area and, therefore, the probability that during the postoperative period patients would be treated elsewhere (in a nonpublic hospital) for a dislocation is very low. Although it could be the case for those who left the area and were lost to follow-up, the number of patients who did so within 12 months after THA was very low (0.9%). Secondly, information regarding the angle of cup inclination was available only for a subgroup of patients. However, mean cup inclination did not substantially differ between the 2 cupsize groups. As our routine postoperative protocol was only an anteroposterior pelvic radiograph, cup and femoral anteversion angles were not available. The latter is a third limitation of our study as anteversion of the
Cup size and dislocation risk after primary total hip arthroplasty Peter et al
femoral stem is a contributing risk factor for dislocation. And, lastly, our results were limited to patients operated on via a transgluteal approach. Although we believe that cup size is an important risk factor for dislocation with any surgical approach, the magnitude of the effect may be different in patients operated on via other approaches.
Conclusion In patients with a hybrid THA and a 28-mm head, we found a 2 times greater risk of dislocation among those with a larger cup diameter (≥56 mm). Therefore, our results support the use of a 28-mm head with smaller cup sizes, whereas with greater cup diameters it might be reasonable to increase the head size to reduce the incidence of dislocation. It should be clear that, although a large femoral head is a help, the best prophylaxis of dislocation is a careful and correct surgical technique.
References 1. Khatod M, Barber T, Paxton E, et al. An analysis of the risk of hip dislocation with a contemporary total joint registry. Clin Orthop Relat Res 2006;447:19. 2. Phillips CB, Barrett JA, Losina E, et al. Incidence rates of dislocation, pulmonary embolism, and deep infection during the first six months after elective total hip replacement. J Bone Joint Surg Am 2003;85-A:20. 3. Turner RS. Postoperative total hip prosthetic femoral head dislocations. Incidence, etiologic factors, and management. Clin Orthop Relat Res 1994;301:196. 4. Lubbeke A, Stern R, Garavaglia G, et al. Differences in outcomes of obese women and men undergoing primary total hip arthroplasty. Arthritis Rheum 2007;57:327. 5. Sadr Azodi O, Adami J, Lindstrom D, et al. High body mass index is associated with increased risk of implant dislocation following primary total hip replacement: 2,106 patients followed for up to 8 years. Acta Orthop 2008;79:141. 6. Bystrom S, Espehaug B, Furnes O, et al. Femoral head size is a risk factor for total hip luxation: a study of 42,987 primary hip arthroplasties from the Norwegian Arthroplasty Register. Acta Orthop Scand 2003;74:514. 7. Ekelund A, Rydell N, Nilsson OS. Total hip arthroplasty in patients 80 years of age and older. Clin Orthop Relat Res 1992;281:101. 8. Jolles BM, Zangger P, Leyvraz PF. Factors predisposing to dislocation after primary total hip arthroplasty: a multivariate analysis. J Arthroplasty 2002;17:282. 9. Kim YH, Choi Y, Kim JS. Influence of patient-, design-, and surgery-related factors on rate of dislocation after primary cementless total hip arthroplasty. J Arthroplasty 2009;24: 1258.
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10. Berry DJ, von Knoch M, Schleck CD, et al. Effect of femoral head diameter and operative approach on risk of dislocation after primary total hip arthroplasty. J Bone Joint Surg Am 2005;87:2456. 11. Burroughs BR, Hallstrom B, Golladay GJ, et al. Range of motion and stability in total hip arthroplasty with 28-, 32-, 38-, and 44-mm femoral head sizes. J Arthroplasty 2005; 20:11. 12. Kelley SS, Lachiewicz PF, Hickman JM, et al. Relationship of femoral head and acetabular size to the prevalence of dislocation. Clin Orthop Relat Res 1998;355:163. 13. Amstutz HC, Le Duff MJ, Beaule PE. Prevention and treatment of dislocation after total hip replacement using large diameter balls. Clin Orthop Relat Res 2004;429:108. 14. Conroy JL, Whitehouse SL, Graves SE, et al. Risk factors for revision for early dislocation in total hip arthroplasty. J Arthroplasty 2008;23:867. 15. Cuckler JM, Moore KD, Lombardi Jr AV, et al. Large versus small femoral heads in metal-on-metal total hip arthroplasty. J Arthroplasty 2004;19:41. 16. Geller JA, Malchau H, Bragdon C, et al. Large diameter femoral heads on highly cross-linked polyethylene: minimum 3-year results. Clin Orthop Relat Res 2006; 447:53. 17. Malik A, Maheshwari A, Dorr LD. Impingement with total hip replacement. J Bone Joint Surg Am 2007;89:1832. 18. Masonis JL, Bourne RB. Surgical approach, abductor function, and total hip arthroplasty dislocation. Clin Orthop Relat Res 2002;405:46. 19. Lee BP, Berry DJ, Harmsen WS, et al. Total hip arthroplasty for the treatment of an acute fracture of the femoral neck: long-term results. J Bone Joint Surg Am 1998;80:70. 20. Lubbeke A, Moons KG, Garavaglia G, et al. Outcomes of obese and nonobese patients undergoing revision total hip arthroplasty. Arthritis Rheum 2008;59:738. 21. Crowninshield RD, Maloney WJ, Wentz DH, et al. Biomechanics of large femoral heads: what they do and don't do. Clin Orthop Relat Res 2004;429:102. 22. Kluess D, Martin H, Mittelmeier W, et al. Influence of femoral head size on impingement, dislocation and stress distribution in total hip replacement. Med Eng Phys 2007; 29:465. 23. Nadzadi ME, Pedersen DR, Callaghan JJ, et al. Effects of acetabular component orientation on dislocation propensity for small-head-size total hip arthroplasty. Clin Biomech (Bristol, Avon) 2002;17:32. 24. Padgett DE, Lipman J, Robie B, et al. Influence of total hip design on dislocation: a computer model and clinical analysis. Clin Orthop Relat Res 2006;447:48. 25. Scifert CF, Brown TD, Pedersen DR, et al. A finite element analysis of factors influencing total hip dislocation. Clin Orthop Relat Res 1998;355:152.
Cup Size and Risk of Dislocation After Primary Total Hip Arthroplasty Robin Peter, MD, Anne Lübbeke, MD, DSc, Richard Stern, MD, and Pierre Hoffmeyer, MD
Abstract: We evaluated the relationship between cup diameter and dislocation risk in patients undergoing primary total hip arthroplasty (THA) with a 28-mm head. There were 50 dislocations, 28 of which occurred in 2221 (1.3%) THAs with a cup diameter smaller than 56 mm and 22 in 513 (4.3%) with a cup diameter of 56 mm or larger. Dislocation risk varied between 0.6% and 2.4% in the smaller cup group and between 4.1% and 5.2% in the larger cup group. The risk was substantially higher in the large cup group (unadjusted odds ratio, 3.5; 95% confidence interval, 2.0-6.2). Multivariable logistic regression revealed an adjusted odds ratio of 2.4 (95% confidence interval, 1.2-4.9). Patients with THA (28-mm head) had more than twice the risk of dislocation with cup size of 56 mm or higher compared to patients with smaller cups. Keywords: total hip dislocation, hip arthroplasty dislocation, acetabular cup size: acetabular cup diameter, total hip head size. © 2011 Elsevier Inc. All rights reserved.
One of the most frequent complications after total hip arthroplasty (THA) is dislocation. Although much effort has been devoted to its prevention, the risk of dislocation after primary THA still remains between 2% and 5% [1-3]. Much has been written concerning the etiology and prevention of dislocation. Circumstances such as the patient's overall condition generally cannot be influenced by the surgeon, and obese patients have been reported to be particularly at risk for dislocation [4,5]. Moreover, it has been demonstrated that older age and higher American Society of Anesthesiologists (ASA) scores are aggravating factors [1,6-9]. The relationship between femoral head size and risk of dislocation has been extensively studied, and the general consensus is that the risk of dislocation is greater with smaller head sizes [1,6,10-12]. In addition, other such technical points as surgical approach and correct positioning and orientation of both the cup and femoral implant are known risk factors for dislocation [8,10,11,13-18]. To the best of our knowledge, only 1
From the Division of Orthopaedics and Trauma Surgery, University Hospitals of Geneva, Geneva, Switzerland. Submitted February 12, 2010; accepted November 27, 2010. The conflict-of-interest statement associated with this article can be found at doi:10.1016/j.arth.2010.11.015. Reprint requests: Richard Stern, MD, Division of Orthopaedics and Trauma Surgery, University Hospitals of Geneva, 4, rue Gabrielle Perret-Gentil, 1211 Geneva, Switzerland. © 2011 Elsevier Inc. All rights reserved. 0883-5403/2608-0030$36.00/0 doi:10.1016/j.arth.2010.11.015
study has specifically addressed the relationship between a larger outer cup diameter and an increased risk of dislocation [12]. Our objective was to evaluate the relationship between cup size and risk of dislocation within a large cohort of patients with the same type of implant, surgical approach, and femoral head size, while taking into account other known risk factors.
Material and Methods Study Population and Study Design Since March 1996, all patients undergoing THA in a university hospital are routinely enrolled in a prospective hospital-based registry and followed longitudinally. For this prospective cohort study, we included all primary hybrid (uncemented cup and cemented stem) THAs performed between March 1996 and July 2008. All patients were operated upon in the lateral decubitus position and the surgical incision was directly lateral. The hip joint was exposed via a standard transgluteal approach, splitting the distal part of the gluteus medius as well as the gluteus minimus off the anterior aspect of the greater trochanter. All patients received an uncemented press-fit titanium mesh backed cup (Morscher cup; Zimmer, Ltd, Winterthur, Switzerland) and a cemented stem (Muller straight stem or Virtec stem, Zimmer) with a 28-mm head. Of 2759 THAs eligible for inclusion, 25 (0.9%) had missing information on cup diameter and were excluded, leaving 2341 patients undergoing 2734 THAs. Ethics committee approval was obtained for this study.
1305
1306 The Journal of Arthroplasty Vol. 26 No. 8 December 2011 Study Variables The exposure of interest was the cup diameter analyzed as a categorical variable (ranging from 44 to 64 mm) and as a dichotomized variable (b56 vs ≥56 mm). The main outcome of interest was the occurrence of dislocation within the first year postoperative. A fall from any level of energy resulting in a dislocation was excluded. This was the case for 5 patients. The following covariates (potential confounding factors) of the effect of cup diameter on the risk of dislocation were measured: (a) age at operation, as continuous and dichotomized variable (b70 vs ≥70 years); (b) sex; (c) weight; (d) height; (e) body mass index (BMI) as continuous and dichotomized variable (b30 vs ≥30 kg/m2); (f) American Society of Anesthesiologists (ASA) score investigated as a binary variable (ASA 1-2 vs 3-4); (g) diagnosis (primary or secondary osteoarthritis vs posttraumatic osteoarthritis); (h) surgeon's experience (junior vs senior surgeon); (i) bearing surface (ceramic-polyethylene vs metal-metal); and (j) cup inclination investigated as a continuous and dichotomized variable (≤45° vs N45°). Data Collection Information about the preoperative status and the surgical intervention was routinely documented by the operating surgeon on specifically designed data collection forms. Any dislocation occurring within the first year after surgery and treated at our hospital, or reported to the orthopaedic surgeon during one of the follow-up visits, was systematically included in the database by a trained data manager. The data entry of any complication (including dislocation) or reoperation in a patient with a hip prosthesis was double checked by the physician in charge of the hip registry (A.L.). Furthermore, we periodically verified any main complication related to THA (including dislocation) by comparing the hospital diagnosis coding system and our medical records. Cup inclination was available for patients with dislocation and for a randomly selected subgroup without the event of interest (overall available for n = 337 THAs). It was measured on postoperative anteroposterior radiographs by a physician blinded to the outcome of interest. Statistical Analysis First, the distribution of baseline characteristics was compared among the 2 exposure groups (cup diameter b56 vs ≥56 mm). To estimate the magnitude of the differences between them, we calculated relative risks and 95% confidence intervals (CI) for dichotomized and mean differences and 95% CIs for continuous variables. Second, to assess the association between cup diameter and risk of dislocation, we performed univariable logistic regression. Then, multivariable logistic regression modeling was used to adjust the effect of cup
diameter on the dislocation risk for confounding variables (covariates). Of all the above-mentioned covariates, only age, BMI, and posttraumatic THA turned out to be the confounding factors in the relationship of interest (P b .05). For the final model, we used the dichotomized version of age and BMI. Although the covariate, sex, was not statistically significant, it was maintained in the final model. Furthermore, height was also maintained because adding it to the model changed the point estimate for the relationship of interest by 10% to 15%. This was not the case for the other covariates such as ASA score, bearing surface, cup inclination, and surgeon experience, which were excluded. Third, the relationship between cup diameter and risk of dislocation taking into account other known risk factors such as older age and obesity was further explored by using stratification. Based on the stratified results, a graph was created illustrating the risk of dislocation in the 2 diameter groups in relation to the other risk factors. Data were missing for BMI and/or ASA score for 47 patients (1.7%). Given the low frequency, no imputations were performed. The final regression analysis was done including 2687 THAs (98.3%).
Results In total, 2734 THAs were included (1215 in men, 1519 in women) in 2341 patients with a mean age of 70.6 years (SD, ±10.2) and a mean BMI of 27.0 kg/m2 (SD, ±4.6). In 2221 THAs (81.2%), a cup with a diameter smaller than 56 mm was used, whereas in 513 THAs (18.8%) a cup with a diameter of 56 mm or higher was used. Larger cups were used almost exclusively in men (women, 3.2% of THAs; men, 38.2% of THAs; Table 1). Patients with larger cup diameters were slightly younger, significantly taller, had a higher mean BMI, and, more often, had an ASA score of 3 to 4. In addition, larger cups were more often used by senior surgeons. A total of 50 dislocations occurred within the first year postoperative, 28 (1.3%) in the 2221 THAs with a diameter smaller than 56 mm and 22 (4.3%) in the 513 THAs with a diameter of 56 mm or higher. The dislocation risk in the smaller diameter group varied between 0.6% and 2.4%, whereas the risk in the larger diameter group varied between 4.1% and 5.2% (Fig. 1). There was a substantially higher risk of dislocation in the group with a cup diameter of 56 mm or higher, with an absolute risk increase of 3% and a relative risk increase of 331%. The unadjusted odds ratio was 3.5 (95% CI 2.0-6.2). Multivariable logistic regression (Table 2) adjusting for age, sex, BMI, height, and diagnosis revealed an adjusted odds ratio of 2.4 (95% CI 1.2-4.9). The risk of dislocation in relation to the other risk factors is illustrated in Fig. 2. Compared to the smaller diameter group, the risk was always higher in the group
Cup size and dislocation risk after primary total hip arthroplasty Peter et al
1307
Table 1. Baseline Characteristics According to the 2 Groups (Cup Diameter b56 and ≥56 mm) Cup Diameter b56 mm (n = 2221 hips) Women Men Age, mean (±SD) BMI, mean (±SD)* Height (cm), mean (±SD)* ASA score † 1-2 3-4 Posttraumatic No Yes Surgeon experience Junior Senior Bearing surface Ceramic-polyethylene Metal-metal Cup inclination, ‡ mean (±SD) Cup inclination, N45° ‡
%
Cup Diameter ≥56 mm (n = 513 hips)
%
Relative Risks (95% CI); Mean Difference (95% CI)
P Value
11.8 (8.9 to 15.7) 1.2 (0.2 to 2.1) −1.0 (−0.6 to −1.5) −10.7 (−9.9 to −11.4)
b.001 .022 b.001 b.001
1470 751 70.9 (±10.2) 26.8 (±4.6) 164.0 (±8.0)
96.8 61.8
49 464 69.7 (±10.0) 27.8 (±4.5) 174.7 (±7.4)
3.2 38.2
1626 565
82.6 77.4
342 165
17.4 22.6
1.3 (1.1 to 1.5)
.002
2152 69
81.3 79.3
495 18
18.7 20.7
1.1 (0.7 to 1.7)
.640
1884 337
82.7 74.1
395 118
17.3 25.9
1.5 (1.3 to 1.8)
b.001
1759 462 38.8° (±6.8) 39°
81.4 80.8
403 110 38.6° (±7.5) 16°
18.6 19.2
1.0 (0.9 to 1.2) 0.2 (−1.6 to 1.9) 1.4 (0.8 to 2.3)
.748 .850 .229
15.0
20.8
* Height/BMI missing for 47 THAs. † ASA score missing for 37 THAs. ‡ Cup inclination available for n = 337 THAs (260 in smaller and 77 in larger cup group).
with the larger cup diameters whether there were additional risk factors present or not. The use of a large cup in the presence of older age and obesity was associated with the highest risk of dislocation of 7.5%, as compared to the lowest risk of 0.6% in the younger, nonobese patients fitted with a smaller cup.
Discussion We observed a significant increase in the risk of dislocation within the first year postoperative in primary THAs using a large cup size and a 28-mm head. Comparing smaller (b56 mm) to larger cup sizes (≥56
mm), we found the risk of dislocation to be more than twice as high in the larger size group (4.3% vs 1.3%). This association remained significant even after taking into account other known risk factors. To the best of our knowledge, only 1 study by Kelley et al [12] has thus far addressed the relationship between THA stability and outer acetabular diameter. Using posterior or lateral approaches, these authors compared the rate of dislocation with both 22-mm and 28-mm head sizes and various cup diameters. In a small sample group of only 30 patients (31 THAs), these authors evaluated 2 different head sizes (22 and
Fig. 1. Risk of dislocation according to cup size.
1308 The Journal of Arthroplasty Vol. 26 No. 8 December 2011 Table 2. Effect of Cup Diameter on Dislocation Adjusted for Relevant Confounding Factors Using Multivariate Logistic Regression Odds Ratio (95% CI) Cup diameter b56 vs ≥56 mm (≥56 = 1) Age b70 vs ≥70 y (≥70 = 1) Sex (female = 1) BMI b30 vs ≥30 kg/m2 (obese = 1) Height (continuous) THA Postfracture (yes = 1)
2.4 (1.2 to 4.9) 2.5 (1.3 to 4.7) 0.8 (0.3 to 1.8) 2.1 (1.2 to 3.9) 1.03 (0.98 to 1.07) 4.7 (1.7 to 12.6)
28 mm) with cup sizes of 54 mm or smaller and 56 mm or larger. With such small subgroups, also including 2 different surgical approaches, it is difficult to extract any meaningful conclusions. However, in a second larger study population (n = 308) utilizing only the posterior approach and including only 28-mm head sizes, the use of an outer diameter of 60 mm or larger was associated with a significantly increased risk of dislocation as compared to an outer diameter smaller than 60 mm (11.3% vs 3.9%, respectively). In contrast to the work of Kelley et al [12], our large sample size allowed us to adjust for other known risk factors. In addition to cup diameter, older age (≥70 years), obesity, and postfracture THA remained as statistically significant risk factors in our final model. Previous studies have already shown a significant increase in the risk of dislocation in association with these factors [4,6,7,9,14,19,20]. However, the risk of dislocation was not influenced by the type of bearing surface, sex, ASA score, surgeon's experience, or angle of cup inclination, the other possible confounding factors evaluated in this study. The relationship between sex and cup diameter merits further discussion. The vast majority of THAs in the larger cup diameter group (464/513, 90%) were implanted in men. This led to a crude higher risk of dislocation in men (data not shown) and was related to the disproportionally greater use of larger cup dia-
Fig. 2. Risk of dislocation stratified by the 2 cup-size groups and by age and obesity.
meters among them as seen in the results of the multivariable regression analysis. After adjusting for cup diameter (and the other covariates remaining in the model), the association between sex and dislocation was no longer significant. Kelley et al [12] showed that a large difference between the size of the femoral head and outer cup diameter (mismatch) further increased the risk of dislocation more than simply cup diameter. They proposed 3 reasons for this, including anatomic mismatch, pseudocapsule attachment, and prosthetic impingement. But they provided no evidence for their hypotheses. As regards the pseudocapsule, it is circumferentially attached to the acetabular rim, but never actually adheres to the polyethylene surrounding the cup bore. With a larger acetabular component the pelvic insertion of this pseudocapsule is at a greater distance from the head, which we assume would allow for a larger joint cavity to accommodate the head in the event of a dislocation. On the other hand, a closer match between head and cup outer diameters would restrict the inner size of the neo-capsule and thus reduce the risk of dislocation. In our search of the literature, we found no biomechanical studies that specifically looked at the effect of cup size on hip stability. There are studies using finite element analyses and computer-aided programs to evaluate certain variables as they relate to hip stability and dislocation [11,21-25], but none of these address the issue of cup size. Strengths and Limitations Our first main strength is that our study was conducted prospectively in a large cohort of patients from a university hospital. Secondly, we restricted inclusion to patients operated upon with use of the same surgical approach, the same head size (28 mm), and the same cup. No hooded liners were used. Thirdly, the large sample size of our study population allowed us to adjust for other main known risk factors in addition to head size and surgical approach. One possible limitation of our study concerns loss of patients in follow-up. However, our university hospital is the only public hospital in the area and, therefore, the probability that during the postoperative period patients would be treated elsewhere (in a nonpublic hospital) for a dislocation is very low. Although it could be the case for those who left the area and were lost to follow-up, the number of patients who did so within 12 months after THA was very low (0.9%). Secondly, information regarding the angle of cup inclination was available only for a subgroup of patients. However, mean cup inclination did not substantially differ between the 2 cupsize groups. As our routine postoperative protocol was only an anteroposterior pelvic radiograph, cup and femoral anteversion angles were not available. The latter is a third limitation of our study as anteversion of the
Cup size and dislocation risk after primary total hip arthroplasty Peter et al
femoral stem is a contributing risk factor for dislocation. And, lastly, our results were limited to patients operated on via a transgluteal approach. Although we believe that cup size is an important risk factor for dislocation with any surgical approach, the magnitude of the effect may be different in patients operated on via other approaches.
Conclusion In patients with a hybrid THA and a 28-mm head, we found a 2 times greater risk of dislocation among those with a larger cup diameter (≥56 mm). Therefore, our results support the use of a 28-mm head with smaller cup sizes, whereas with greater cup diameters it might be reasonable to increase the head size to reduce the incidence of dislocation. It should be clear that, although a large femoral head is a help, the best prophylaxis of dislocation is a careful and correct surgical technique.
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