Surgical approach and BMI were the only variables that were indicators for malpositioning of abduction, version, and combined abduction and version in the multivariate analyses (Table 3). Head size and surgeon volume were also variables identified in the multivariate analyses. Independent predictors for malpositioning in abduction only or in abduction and version combined were BMI (categorized as obese versus not obese), surgical approach, and surgeon volume. For version alone, head size replaced surgeon volume as an independent indicator of angles outside the acceptable range. The most accurate subcategories within each independent predictor used to create odds ratios were not obese for BMI, less than 32 mm for head size, high volume for surgeon volume, and posterolateral for approach. The odds ratios are the increased risk of malpositioning in abduction, version, or both based on the category of that prediction factor (Table 4). Using the three variables of BMI, surgical approach, and surgeon volume from the combined abduction and version analysis, we examined the percent chance of malpositioning for every possible combination of these three variables’ subcategories (Table 5).
Malpositioning of the acetabular cup has been linked to an increased rate of dislocation, liner fracture, and increased wear. The increased dislocation risk has been well established [20, 23, 26, 54], and implant wear is greater for malpositioned cups with metal-on-polyethylene, metal-on-metal, and ceramic-on-ceramic bearings [11, 14, 25]. Specifically, one study showed surgical technique factors such as abduction angle of the cup were the most predictive of polyethylene wear . This is possibly more important with hard-on-hard bearings due to the increased effects of edge loading in a rigid construct. Metal-on-metal and ceramic-on-ceramic bearings have wear rates 10 to 30 times as high when acetabular cup angles exceeded 55°, measured in vitro on simulators and in vivo by serum ion levels and implant retrieval studies [11, 32]. While the effects of cup positioning on complications are well understood, a comprehensive analysis of patient and surgical factors contributing to poor implant positioning is lacking, with the exception of the study by Bosker et al. , which is limited by its relatively small sample size. We determined percentages of optimally positioned acetabular cups based on various patient and surgical factors, and then determined which of those factors, if any, correlated to the orientation of the acetabular cup in a large group of THA patients.
There are several limitations to our study. First is the use of a computer software program to obtain inclination and version angles without accounting for pelvic tilt, which could influence the calculated angles. Recent studies show CT scans measure acetabular version more accurately than radiographs  and the combination of CT scans and radiographic information is the most effective tool to accurately measure cup position [31, 43, 51, 57]. Second, we did not measure the rotational position of the femoral stem in the femoral canal. This is relevant because cups that are outside the limits of the acceptable version ranges may have been purposely placed there to align with the femoral component, especially in revision cases. Third, the study was conducted at a single, high-volume teaching hospital. Since surgery volume has been correlated to cup positioning, our observations could differ at a smaller, lower-volume hospital. Fourth, the optimal ranges we used were based on literature [23, 54] and surgeon input. Many studies cite 50° as the upper limit for optimal cup abduction. The upper limit for this study was set at 45° based on a recent study indicating increased wear in cups with hard-on-hard bearings placed with abduction angles greater than 45° . Also, to combine readings of cup abduction from mDesk™ and HAS, a correlation between a test set of readings was performed. The average difference of 1.66° is a clinically acceptable variation, and a Pearson R of 0.99 showed both programs can be used interchangeably to determine angles. This justified the inclusion of the 39 hips that could only be read using mDesk™. Finally, we did not examine the long-term clinical implications of our observations.
Average abduction and version angles of 42.2° and 12.7° were similar to those reported in the literature which range from 37.5° to 49.7° for abduction and 10.7° to 27.3° for version (Table 6) [5, 24, 36, 44, 45, 48, 53]. The literature inconsistently reports acceptable angle ranges for optimally positioned cups, and the percentage of acceptably placed cups in both abduction and version using the most widely used acceptable range (the Lewinnek range ) varies from 70.5%  to 25.7% . We found that 47% of cups were properly positioned in both, which is slightly lower than the best accuracy presented in the literature, however, we used a slightly tighter abduction range criteria (30-45°) compared to the Lewinnek range (30-50°). We found a similar percentage of acetabular cups within the ranges for abduction (62%) and version (79%) alone compared to the most accurate study in the literature with 85% and 83% optimally positioned in abduction and version respectively .
Surgical approach was the only factor indicating cup malpositioning in every analysis. This is in direct contrast to two previously published studies [5, 36] which show no link between surgical approach and cup positioning. In this study, the posterolateral approach was the most used and was 20% more accurate than all other approaches. It is possible some confounding factors contributed to this finding in the univariate analyses because the posterolateral approach was used predominantly by the highest-volume surgeon. However, the second high-volume surgeon did not use the posterolateral approach, and our findings that surgical approach was an independent predictor in the multivariate model indicated that even among high volume surgeons, surgical approach still affects cup malpositioning. The binary logistic regression used in the multivariate approach accounts for other variables, including surgeon volume. The less invasive MIS approach was the least accurate, with anterolateral and posterolateral approaches having similar accuracies. The inaccuracies of the MIS approach could be caused by a more constrained working space and decreased direct vision associated with MIS, making the accurate assessment of anatomy and placement of the acetabular cup more difficult compared to other approaches. The volume of surgeries performed was another indicator of malpositioning in abduction and abduction or version combined. Three previous studies examined the effect of surgical volume and experience on cup positioning, two of which reported no affect of surgical experience [24, 45] while one found a substantial difference between surgeons compared to their residents . Two of these studies were limited by a small sample size [5, 36], and the third examined only surgeon qualifications, implanted model, and side . For surgeon volume in our study, the high-volume surgeons who performed an average of 164 THAs per year were 16% more accurate than the low-volume surgeons who performed an average of 13 THA’s per year. This indicates that a lower-volume surgeon would benefit the most from additional training or the use of some form of navigational assistance. Previous studies have established a link between surgeon volume and infection, dislocation, revision, or complications [30, 49, 52]; therefore, the link between proper cup positioning and surgeon volume is not unexpected. Also, while we found an independent link between surgeon volume and cup positioning in abduction and version combined, the surgeon experience could prove even more important if these experienced surgeons were performing more difficult procedures on more complex patients. Surgeon volume was not an independent indicator of attaining an acceptable version angle. This suggests a lower-volume surgeon’s greater risk of cup malpositioning is due to a lack of accuracy in cup abduction, not cup version. This discrepancy could be due to the use of the lateral patient position for all surgeries except for those using the two-incision MIS approach, resulting in a higher variation in pelvic tilt and lower variation in pelvic flexion. A previously mentioned study of acetabular cup placement by orthopaedic surgeons and residents found a difference between intended and actual angles for abduction but not for version . This supports our finding that performing a greater volume of surgeries can increase accuracy of cup placement in abduction but not necessarily in version. BMI was another indicator for increased risk of cup malpositioning. More specifically, obesity had a greater risk of malpositioning than the other BMI categories of underweight to overweight. This contradicts three previous studies indicating no relationship between cup position and BMI, but those studies were also limited by their small sample size [5, 44, 53]. A possible explanation for the decreased accuracy found in positioning cups in obese patients is the relatively smaller field for a given incision size due to the increased amount of adipose tissue. The excess tissue can also make it more difficult to locate anatomic landmarks. The link between obesity and malpositioning of the acetabular cup established in this study could explain some of the increased incidence of post-THA prosthesis dislocation found in obese populations in other studies [27, 46]. In addition to surgeon volume, surgical approach, and BMI, which were indicators in multiple analyses, femoral head size was an independent indicator of cup malpositioning for version alone. Head size was also a predictor of implant malpositioning in abduction and version when other variables were not taken into account. There are currently no studies comparing head size or cup outer diameter to positioning of the cup. Cups paired with femoral head sizes smaller than 32 mm were more accurately placed, with 9% greater accuracy than 32-mm heads and 22% greater accuracy than large heads. Since most surgeons are aware a larger head size allows for greater ROM and a decreased risk of impingement and dislocation, extra attention to acetabular cup positioning could be lacking when larger head sizes are used. Because of this, surgeons should be particularly aware of this tendency.
As a result of this study, we are currently reporting cup positioning back to surgeons on a regular basis. This monthly analysis of each surgeon’s cup abduction and version angles has the potential to increase cup position accuracy, allowing them to continuously correct for tendencies to over- or under rotate their cups. Regular feedback is facilitated by the presence of a registry and the ability to consistently evaluate cup positioning using a database of radiographs and associated patient factors. With the ultimate goal of increasing the accuracy of acetabular cup position to reduce the risk of complications, we identified the main factors contributing to malpositioning of the cup, namely volume of surgeries performed, surgical approach, and obesity. Related to surgical approach, special consideration should be taken when positioning and fixing the patient on the operating table. Navigational assistance increases the accuracy of acetabular cup positioning regardless of surgical experience or approach [8, 19, 37, 40, 55]. The use of navigation could have a positive impact on patients, especially for surgeons who perform fewer THAs per year or use a MIS approach, indicating a need for less expensive, more accessible navigation options. Our data suggest special care should be taken when positioning cups in obese patients, especially when the surgery is being performed by a low-volume surgeon using the MIS approach. Further analyses on patient and surgical factors’ influence on cup position at a lower volume medical center would provide a valuable comparison.
1. Ali Khan, MA. and Brakenbury, PH.IS R Dislocation following total hip replacement. J Bone Joint Surg Br
1981; 63: 214-218.
2. Arthursson, AJ., Furnes, O., Espehaug, B., Havelin, LI. and Soreide, JA. Prosthesis survival after total hip arthroplasty—does surgical approach matter? Analysis of 19,304 Charnley and 6,002 Exeter primary total hip arthroplasties reported to the Norwegian Arthroplasty Register. Acta Orthop
2007; 78: 719-729. 10.1080/17453670710014482
3. Bartz, RL., Noble, PC., Kadakia, NR. and Tullos, HS. The effect of femoral component head size on posterior dislocation of the artificial hip joint. J Bone Joint Surg Am
2000; 82: 1300-1307.
4. Biedermann, R., Tonin, A., Krismer, M., Rachbauer, F., Eibl, G. and Stockl, B. Reducing the risk of dislocation after total hip arthroplasty: the effect of orientation of the acetabular component. J Bone Joint Surg Br
2005; 87: 762-769. 10.1302/0301-620X.87B6.14745
5. Bosker, BH., Verheyen, CC., Horstmann, WG. and Tulp, NJ. Poor accuracy of freehand cup positioning during total hip arthroplasty. Arch Orthop Trauma Surg
2007; 127: 375-379. 10.1007/s00402-007-0294-y
6. Bradford, L., Kurland, R., Sankaran, M., Kim, H., Pruitt, LA. and Ries, MD. Early failure due to osteolysis associated with contemporary highly cross-linked ultra-high molecular weight polyethylene. A case report. J Bone Joint Surg Am
2004; 86: 1051-1056.
7. Bragdon, CR., Martell, JM., Greene, ME., Estok, DM, II, Thanner, J., Karrholm, J., Harris, WH. and Malchau, H. Comparison of femoral head penetration using RSA and the Martell method. Clin Orthop Relat Res
2006; 448: 52-57. 10.1097/01.blo.0000224018.88410.83
8. Broers, H. and Jansing, N. How precise is navigation for minimally invasive surgery? Int Orthop
2007; 31: (Suppl 1):S39-S42. 10.1007/s00264-007-0431-9
9. Colwell, CWJ. Instability after total hip arthroplasty. Current Orthop Prac
2009; 20: 8-14. 10.1097/BCO.0b013e3181926d7d
10. Conroy, JL., Whitehouse, SL., Graves, SE., Pratt, NL., Ryan, P. and Crawford, RW. Risk factors for revision for early dislocation in total hip arthroplasty. J Arthroplasty
2008; 23: 867-872. 10.1016/j.arth.2007.07.009
11. Haan, R., Pattyn, C., Gill, HS., Murray, DW., Campbell, PA. and Smet, K. Correlation between inclination of the acetabular component and metal ion levels in metal-on-metal hip resurfacing replacement. J Bone Joint Surg Br
2008; 90: 1291-1297. 10.1302/0301-620X.90B10.20533
12. Dorr, LD. Acetabular cup position: the imperative of getting it right. Orthopedics
2008; 31: 898-899. 10.3928/01477447-20080901-10
13. Enocson, ATJ., Tornkvist, H. and Lapidus, L. Dislocation of hemiarthroplasty after femoral neck fracture: better outcome after the anterolateral approach in a prospective cohort study on 739 consecutive hips. Acta Orthop
2008; 79: 211-217. 10.1080/17453670710014996
14. Gallo, J., Havranek, V. and Zapletalova, J. Risk factors for accelerated polyethylene wear and osteolysis in ABG I total hip arthroplasty. Int Orthop
2010; 34: 19-26. 10.1007/s00264-009-0731-3
15. Ghelman, B., Kepler, CK., Lyman, S. and Della Valle, AG.CT outperforms radiography for determination of acetabular cup version after THA. Clin Orthop Relat Res
2009; 467: 2362-2370. 10.1007/s11999-009-0774-1
16. Holt, G., Murnaghan, C., Reilly, J. and Meek, RM. The biology of aseptic osteolysis. Clin Orthop Relat Res
2007; 460: 240-252.
17. Hui, AJ., McCalden, RW., Martell, JM., MacDonald, SJ., Bourne, RB. and Rorabeck, CH. Validation of two and three-dimensional radiographic techniques for measuring polyethylene wear after total hip arthroplasty. J Bone Joint Surg Am
2003; 85: 505-511.
18. Jasty, M., Goetz, DD., Bragdon, CR., Lee, KR., Hanson, AE., Elder, JR. and Harris, WH. Wear of polyethylene acetabular components in total hip arthroplasty. An analysis of one hundred and twenty-eight components retrieved at autopsy or revision operations. J Bone Joint Surg Am
1997; 79: 349-358. 10.1302/0301-620X.79B3.7872
19. Jolles, BM., Genoud, P. and Hoffmeyer, P. Computer-assisted cup placement techniques in total hip arthroplasty improve accuracy of placement. Clin Orthop Relat Res
2004; 426: 174-179. 10.1097/01.blo.0000141903.08075.83
20. Kelley, SS., Lachiewicz, PF., Hickman, JM. and Paterno, SM. Relationship of femoral head and acetabular size to the prevalence of dislocation. Clin Orthop Relat Res
1998; 355: 163-170. 10.1097/00003086-199810000-00017
21. Kennedy, JG., Rogers, WB., Soffe, KE., Sullivan, RJ., Griffen, DG. and Sheehan, LJ. Effect of acetabular component orientation on recurrent dislocation, pelvic osteolysis, polyethylene wear, and component migration. J Arthroplasty
1998; 13: 530-534. 10.1016/S0883-5403(98)90052-3
22. Kluess, D., Martin, H., Mittelmeier, W., Schmitz, KP. and Bader, R. Influence of femoral head size on impingement, dislocation and stress distribution in total hip replacement. Med Eng Phys
2007; 29: 465-471. 10.1016/j.medengphy.2006.07.001
23. Kummer, FJ., Shah, S., Lyer, S. and DiCesare, PE. The effect of acetabular cup orientations on limiting hip rotation. J Arthroplasty
1999; 14: 509-513. 10.1016/S0883-5403(99)90110-9
24. Leichtle, U., Gosselke, N., Wirth, CJ. and Rudert, M. Radiologic evaluation of cup placement variation in conventional total hip arthroplasty [in German]. Rofo
2007; 179: 46-52.
25. Leslie, IJ., Williams, S., Isaac, G., Ingham, E. and Fisher, J. High cup angle and microseparation increase the wear of hip surface replacements. Clin Orthop Relat Res
2009; 467: 2259-2265. 10.1007/s11999-009-0830-x
26. Lewinnek, GE., Lewis, JL., Tarr, R., Compere, CL. and Zimmerman, JR. Dislocations after total hip-replacement arthroplasties. J Bone Joint Surg Am
1978; 60: 217-220.
27. Lubbeke, A., Moons, KG., Garavaglia, G. and Hoffmeyer, P. Outcomes of obese and nonobese patients undergoing revision total hip arthroplasty. Arthritis Rheum
2008; 59: 738-745. 10.1002/art.23562
28. Mallory, TH., Lombardi, AV, Jr, Fada, RA., Herrington, SM. and Eberle, RW. Dislocation after total hip arthroplasty using the anterolateral abductor split approach. Clin Orthop Relat Res
1999; 358: 166-172. 10.1097/00003086-199901000-00020
29. Malviya, A. and Holland, JP. Pseudotumours associated with metal-on-metal hip resurfacing: 10-year Newcastle experience. Acta Orthop Belgica
2009; 75: 477-483.
30. Manley, M., Ong, K., Lau, E. and Kurtz, SM. Effect of volume on total hip arthroplasty revision rates in the United States Medicare population. J Bone Joint Surg
2008; 90: 2446-2451. 10.2106/JBJS.G.01300
31. Marx, A., Knoch, M., Pfortner, J., Wiese, M. and Saxler, G. Misinterpretation of cup anteversion in total hip arthroplasty using planar radiography. Arch Orthop Trauma Surg
2006; 126: 487-492. 10.1007/s00402-006-0163-0
32. Morlock, MM., Bishop, N., Zustin, J., Hahn, M., Ruther, W. and Amling, M. Modes of implant failure after hip resurfacing: morphological and wear analysis of 267 retrieval specimens. J Bone Joint Surg Am
2008; 90: (Suppl 3):89-95. 10.2106/JBJS.H.00621
33. Morrey, BF. Difficult complications after hip joint replacement: dislocation. Clin Orthop Relat Res
1997; 344: 179-187. 10.1097/00003086-199711000-00019
34. Morrey, BF. Instability after total hip arthroplasty. Orthop Clin North Am
1992; 23: 237-248.
35. Muratoglu, OK., Bragdon, CR., O’Connor, D., Perinchief, RS., Estok, DM., Jasty, M. and Harris, WH. Larger diameter femoral heads used in conjunction with a highly cross-linked ultra-high molecular weight polyethylene: a new concept. J Arthroplasty.
2001; 16: (8 Suppl 1):24-30.
36. Myers, GJ., Morgan, D., McBryde, CW. and O’Dwyer, K. Does surgical approach influence component positioning with Birmingham Hip Resurfacing? Int Orthop
2009; 33: 59-63. 10.1007/s00264-007-0469-8
37. Najarian, BC., Kilgore, JE. and Markel, DC. Evaluation of component positioning in primary total hip arthroplasty using an imageless navigation device compared with traditional methods. J Arthroplasty
2009; 24: 15-21. 10.1016/j.arth.2008.01.004
38. Nevelos, JE., Ingham, E., Doyle, C., Nevelos, AB. and Fisher, J. The influence of acetabular cup angle on the wear of “BIOLOX Forte” alumina ceramic bearing couples in a hip joint simulator. J Mater Sci
2001; 12: 141-144.
39. Newington, DP., Bannister, GC. and Fordyce, M. Primary total hip replacement in patients over 80 years of age. J Bone Joint Surg Br
1990; 72: 450-452.
40. Nogler, M., Mayr, E., Krismer, M. and Thaler, M. Reduced variability in cup positioning: the direct anterior surgical approach using navigation. Acta orthop
2008; 79: 789-793. 10.1080/17453670810016867
41. Pandit, H., Glyn-Jones, S., McLardy-Smith, P., Gundle, R., Whitwell, D., Gibbons, CL., Ostlere, S., Athanasou, N., Gill, HS. and Murray, DW. Pseudotumours associated with metal-on-metal hip resurfacings. J Bone Joint Surg Br
2008; 90: 847-851. 10.1302/0301-620X.90B7.20213
42. Pedersen, DR., Callaghan, JJ. and Brown, TD. Activity-dependence of the “safe zone” for impingement versus dislocation avoidance. Med Eng Phys
2005; 27: 323-328. 10.1016/j.medengphy.2004.09.004
43. Penney, GP., Edwards, PJ., Hipwell, JH., Slomczykowski, M., Revie, I. and Hawkes, DJ. Postoperative calculation of acetabular cup position using 2-D-3-D registration. IEEE Trans Biomed Eng
2007; 54: 1342-1348. 10.1109/TBME.2007.890737
44. Pirard, E. and Lint, JA. Anteversion of the acetabular component in obese patients. Hip Int
2007; 17: 99-103.
45. Reize, P., Geiger, EV., Suckel, A., Rudert, M. and Wulker, N. Influence of surgical experience on accuracy of acetabular cup positioning in total hip arthroplasty. Am J Orthop (Belle Mead NJ)
2008; 37: 360-363.
46. Sadr Azodi, O., Adami, J., Lindstrom, D., Eriksson, KO., Wladis, A. and Bellocco, R. 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: 141-147. 10.1080/17453670710014897
47. Sanchez-Sotelo, J. and Berry, DJ. Epidemiology of instability after total hip replacement. Orthop Clin North Am
2001; 32: 543-552. 10.1016/S0030-5898(05)70225-X
48. Saxler, G., Marx, A., Vandevelde, D., Langlotz, U., Tannast, M., Wiese, M., Michaelis, U., Kemper, G., Grutzner, PA., Steffen, R., Knoch, M., Holland-Letz, T. and Bernsmann, K. The accuracy of free-hand cup positioning—a CT based measurement of cup placement in 105 total hip arthroplasties. Int Orthop
2004; 28: 198-201. 10.1007/s00264-004-0542-5
49. Shervin, N., Rubash, HE. and Katz, JN. Orthopaedic procedure volume and patient outcomes: a systematic literature review. Clin Orthop Relat Res
2007; 457: 35-41. 10.1097/BLO.0b013e3180375514
50. Shon, WY., Baldini, T., Peterson, MG., Wright, TM. and Salvati, EA. Impingement in total hip arthroplasty a study of retrieved acetabular components. J Arthroplasty
2005; 20: 427-435. 10.1016/j.arth.2004.09.058
51. Shon, WY., Gupta, S., Biswal, S., Hur, CY., Jajodia, N., Hong, SJ. and Myung, JS. Validation of a simple radiographic method to determine variations in pelvic and acetabular cup sagittal plane alignment after total hip arthroplasty. Skeletal Radiol
2008; 37: 1119-1127. 10.1007/s00256-008-0550-4
52. Solomon, DH., Losina, E., Baron, JA., Fossel, AH., Guadagnoli, E., Lingard, EA., Miner, A., Phillips, CB. and Katz, JN. Contribution of hospital characteristics to the volume-outcome relationship: dislocation and infection following total hip replacement surgery. Arthritis Rheum
2002; 46: 2436-2444. 10.1002/art.10478
53. Todkar, M. Obesity does not necessarily affect the accuracy of acetabular cup implantation in total hip replacement. Acta Orthop Belg
2008; 74: 206-209.
54. Widmer, KH. and Zurfluh, B. Compliant positioning of total hip components for optimal range of motion. J Orthop Res
2004; 22: 815-821. 10.1016/j.orthres.2003.11.001
55. Wixson, RL. and MacDonald, MA. Total hip arthroplasty through a minimal posterior approach using imageless computer-assisted hip navigation. J Arthroplasty
2005; 20: (7 Suppl 3):51-56. 10.1016/j.arth.2005.04.024
56. Yamaguchi, M., Akisue, T., Bauer, TW. and Hashimoto, Y. The spatial location of impingement in total hip arthroplasty. J Arthroplasty
2000; 15: 305-313. 10.1016/S0883-5403(00)90601-6
57. Zheng, G., Steppacher, S., Zhang, X. and Tannast, M. Precise estimation of postoperative cup alignment from single standard X-ray radiograph with gonadal shielding. Med Image Comput Comput Assist Interv Int Conf Med Image Comput Comput Assist Interv
2007; 10: (Pt 2):951-959.