Precise reconstruction of hip biomechanics is one goal of THA surgery. Several studies have documented the importance of the hip COR to both longevity of the hip replacement [10, 18, 19, 26-28] and muscle function [1, 9, 17]. In studies on function, the relationship of COR to the hip offset was documented. In clinical series, superomedial or superolateral displacement of the hip COR contributed to loosening of the socket and femur. Superior displacement of COR by 5 mm or more distorts the femoral offset so a high offset stem is required to restore femoral bone position . The use of a high offset stem is consistent with our finding of the necessity of increased offset with superior COR. With our use of computer navigation the data correlated the quantitative change in COR to the quantitative change in hip offset necessary to avoid impingement by intraoperative manual testing. We also investigated whether the quantitative knowledge provided by computer navigation allowed us to obtain offset of the THA within 6 mm of a contralateral normal hip as measured radiographically. This is important because one study correlates offset more than this with increased wear . We were able to keep offset within 6 mm in 78 of 82 hips (95%), and only in hips with a neck shaft angle greater than 135° were we not able to maintain offset within 6 mm (Fig. 7). Finally, we asked whether we could keep radiographic hip length within 6 mm, which has been the standard for avoidance of clinical leg length difference [16, 20]. We were able to do so in 81 of 82 patients (99%), and in these patients the clinical leg lengths were within ± 2.5 mm. In our experience, patients do not tolerate clinical leg length difference more than 3 mm so this number is clinically more important than the radiographic discrepancy of 6 mm. With this biomechanical reconstruction, there was no patient with functional leg length difference , dislocation, or impingement pain .
The most important limitation was the inability to control rotation of the hip for accurate hip offset measurement on radiographs. This potential error was minimized by using the contralateral hip as the control. The second limitation was that validation of the computer navigation software was not performed with CT scans. However, the accuracy of imageless computer navigation for offset and hip length was validated by Renkawitz et al.  in a study of 17 cadavers using CT scans. The true offset was known by measurement of the cadaver and there was close agreement of CT scan and imageless computer navigation. Therefore, imageless computer navigation can be considered accurate in measurement of offset.
We contend reconstruction of offset by THA can only be accurately measured by including the acetabular reconstruction, which usually changes the center of rotation of the hip. This was validated by our correlation of change in the cup COR to offset reconstruction. It clinically confirms the computer model of Kurtz et al. that showed change in acetabular COR had a greater effect on bony impingement than femoral offset . Their computer modeling study showed the greater the variance of the cup center of rotation (more superior, more medial, or both) the more the hip offset (combination of femoral offset and cup offset in their study) had to be increased to avoid impingement. This is the same finding as in our clinical study and consistent with the necessity for high offset stems in one study where 42 of 109 (39%) hips had a hip center superior by a mean 5 ± 6 mm and medially displaced by a mean 3.7 ± 6 mm . The influence of center of rotation was also studied in both surface and conventional THAs . In this study the cup was medialized a mean 6 mm with both types of arthroplasty. With the surface replacement, the difference could not be compensated by increasing femoral offset so medialization caused a reduced hip offset compared to the normal side in 20 of 22 hips. The conventional THAs had an increased offset of mean 5.2 mm in 17 of 19 hips because the femoral component could be adjusted.
Superior displacement of the hip COR decreases the perpendicular distance to the vector of the gluteus medius with consequent reduction of abductor strength [1, 19]. Therefore, the offset must be increased to adjust this perpendicular distance for function. However, increasing the offset more than 5 mm from the normal hip increases polyethylene wear . Therefore, if the hip center is moved by 5 mm or more superiorly, the hip offset must be increased, yet this may reduce the longevity of the hip if offset is increased by more than 5 mm. Our data show that only by limiting superior displacement of the hip COR to 3 mm, and medialization to 5 mm or less, can an increase in offset always be kept within 5 mm (0.4 ± 3.8 mm). These data are compelling evidence that to maximize muscle function and minimize wear the superior COR must be kept within 3 mm of normal. This quantitative correlation confirms the clinical data that correlated failure of THA to radiographic COR displacement of 2 mm or more superiorly [10, 18, 19, 26, 27].
Computer navigation provided intraoperative knowledge of quantitative change of hip offset. Radiographic measurement of hip offset showed more change than measured intraoperatively with navigation because of rotation and magnification of radiographs (Table 1). Using computer navigation, the greater trochanter method of measurement was more likely precise for both hip offset and hip length because it measures change closer to the center of the hip . Distal femoral pin sites cause discomfort for some patients as long as 6 weeks postoperatively, although we experienced no fractures and no sequelae at 1 year. As a consequence of this study, and the published relationship of center of rotation to wear and longevity, we strive to keep the center at no more than 3 mm superior and 5 mm medially. However, the COR of the cup cannot be restored to the COR of the anatomic acetabulum with some hip deformities, such as severe dysplasia or severe migration of the femoral head. Technically, the center of rotation of the cup is determined by the depth of reaming. Without guidance from computer navigation, the best anatomic landmarks are to not ream medially beyond the cortical bone of the cotyloid notch and inferomedially maintain the edge of the cup at the level of the transverse acetabular ligament. At the superoanterior rim of the bony acetabulum the metal shell should be flush with bone. There may be up to 3 mm of metal uncovered by the superior posterior rim of acetabulum because the anatomic acetabulum has average inclination of 55° and cup inclination must not exceed 45° .
Our data educate the orthopaedic surgeon to the importance of the strong correlation of hip offset to COR of the cup; thinking about hip offset rather than femoral offset; and the benefit of computer navigation in maintaining hip offset and length within 6 mm of normal, as measured radiographically. It is important to remember that asymmetric hip offset does occur and restoration to normal may not equal the opposite hip. Additionally, dysplastic hips often require superior displacement of the COR to allow coverage of the cup, which we experienced in both the dysplastic hips in this study. If it is necessary to increase the offset, as we observed in valgus hips, we release that portion of attachment of the iliopsoas muscle to the proximal side of the lesser trochanter, which recesses this muscle and prevents tendinitis .
We thank Patricia J. Paul for preparation of this manuscript.
1. Asayama, I., Chamnongkich, S., Simpson, KJ., Kinsey, TL. and Mahoney, OM. Reconstructed hip joint position and abductor muscle strength after total hip arthroplasty. J Arthroplasty.
2005; 20: 414-420. 10.1016/j.arth.2004.01.016
2. Bourne, RB. and Rorabeck, CH. Soft tissue balancing: The hip. J Arthroplasty
2002; 17: (Suppl 1):17-22. 10.1054/arth.2002.33263
3. Charles, MN., Bourne, RB., Davey, JR., Greenwald, AS., Morrey, BF. and Rorabeck, CH. Soft-tissue balancing of the hip: the role of femoral offset restoration. J Bone Joint Surg Am.
2004; 86: 1078-1088.
4. Dolhain, P., Tsigaras, H., Bourne, RB., Rorabeck, CH., Mac Donald, S. and Mc Calden, R. The effectiveness of dual offset stems in restoring offset during total hip replacement. Acta Orthop Belg
2002; 68: 490-499.
5. Dorr, LD. Hip Arthroplasty: Minimally Invasive Techniques and Computer Navigation
, 1st ed. Philadelphia, PA: WB Saunders; 2005.
6. Flecher, X., Parratte, S., Brassart, N., Aubaniac, JM. and Argenson, JN. Evaluation of the hip center in total hip arthroplasty for old development dysplasia. J Arthroplasty.
2008; 23: 1189-1196. 10.1016/j.arth.2007.10.008
7. Harris, WH. Traumatic arthritis of the hip after dislocation and acetabular fractures: treatment by mold arthroplasty. An end-result study using a new method of result evaluation. J Bone Joint Surg Am.
1969; 51: 737-755.
8. Heaton, K. and Dorr, LD. Surgical release of iliopsoas tendon for groin pain after total hip arthroplasty. J Arthroplasty.
2002; 17: 779-781. 10.1054/arth.2002.33570
9. Johnston, RC., Brand, RA. and Crownenshield, RD. Reconstruction of the hip. A mathematical approach to determine optimum geometric relationships. J Bone Joint Surg Am.
1979; 61: 639-652.
10. Karachalios, T., Hartofilakidis, G., Zacharakis, N. and Tsekoura, M. A 12- to 18-year radiographic follow-up study of Charnley low-friction arthroplasty. The role of the center of rotation. Clin Orthop Relat Res.
1993; 296: 140-147.
11. Krishnan, SP., Carrington, RW., Mohiyaddin, S. and Garlick, N. Common misconceptions of normal hip joint relations on pelvic radiographs. J Arthroplasty.
2006; 21: 409-412. 10.1016/j.arth.2005.10.021
12. Kurtz, WB., Ecker, TM., Reichmann, WM. and Murphy, SB. Factors affecting bony impingement in hip arthroplasty. J Arthroplasty.
2010; 25: 624-634. 10.1016/j.arth.2009.03.024
13. Little, NJ., Busch, CA., Gallagher, JA., Rorabeck, CH. and Bourne, RB. Acetabular polyethylene wear and acetabular inclination and femoral offset. Clin Orthop Relat Res.
2009; 467: 2895-2900. 10.1007/s11999-009-0845-3
14. Long, WT., Dastane, M., Harris, MJ., Wan, Z. and Dorr, LD. Failure of the Durom Metasul acetabular component. Clin Orthop Relat Res.
2010; 468: 400-405. 10.1007/s11999-009-1071-8
15. Loughead, JM., Chesney, D., Holland, JP. and McCaskie, AW. Comparison of offset in Birmingham hip resurfacing and hybrid total hip arthroplasty. J Bone Joint Surg Br.
2005; 87: 163-166. 10.1302/0301-620X.87B2.15151
16. Malik, A., Maheshwari, A. and Dorr, LD. Impingement with total hip replacement. J Bone Joint Surg Am.
2007; 89: 1832-1842. 10.2106/JBJS.F.01313
17. McGrory, BJ., Morrey, BJ., Cahalan, TD., An, KN. and Cabanela, ME. Effect of femoral offset on range of motion and abductor muscle strength after total hip arthroplasty. J Bone Joint Surg Br.
1995; 77: 865-869.
18. Pagnano, MW., Hanssen, AD., Lewallen, DG. and Shaughnessy, WJ. Effect of superior placement of the acetabular component on the rate of loosening after total hip arthroplasty. J Bone Joint Surg Am.
1996; 78: 1004-1014.
19. Ranawat, CS., Dorr, LD. and Inglis, AE. Total hip arthroplasty in protusio acetabuli of rheumatoid arthritis. J Bone Joint Surg Am.
1980; 62: 1059-1065.
20. Ranawat, CS., Rao, RR., Rodriguez, JA. and Bhende, HS. Correction of limb-length inequality during total hip arthroplasty. J Arthroplasty.
2001; 16: 715-720. 10.1054/arth.2001.24442
21. Ranawat, CS. and Rodriguez, JA. Functional leg-length inequality following total hip arthroplasty. J Arthroplasty.
1997; 12: 359-364. 10.1016/S0883-5403(97)90190-X
22. Renkawitz, T., Schuster, T., Herold, T., Goessmann, H., Sendtner, E., Grifka, J. and Kalteis, T. Measuring leg length and offset with an imageless navigation system during total hip arthroplasty: is it really accurate? Int J Med Robot.
2009; 5: 192-197.
23. Resnick, D. Bone and Joint Imaging
, 3rd ed. Philadelphia, PA: Elsevier Saunders; 2005.
24. Sakalkale, DP., Sharkey, PF., Eng, K., Hozack, WJ. and Rothman, RH. Effect of femoral component offset on polyethylene wear in total hip arthroplasty. Clin Orthop Relat Res.
2001; 388: 125-134. 10.1097/00003086-200107000-00019
25. Silva, M., Lee, KH., Heisel, C., Dela Rosa, MS. and Schmalzried, TP. The biomechanical results of total hip resurfacing arthroplasty. J Bone Joint Surg Am
2004; 86-A: 40-60.
26. Stans, AA., Pagnano, MW., Shaughnessy, WJ. and Hanssen, AD. Results of total hip arthroplasty for Crowe type III developmental hip dysplasia. Clin Orthop Relat Res.
1998; 348: 149-157. 10.1097/00003086-199803000-00024
27. Sugano, N., Noble, PC. and Kamaric, E. Predicting the position of the femoral head center. J Arthroplasty.
1999; 14: 102-107. 10.1016/S0883-5403(99)90210-3
28. Yoder, SA., Brand, RA., Pedersen, DR. and O’Gorman, TW. Total hip acetabular component position affects component loosening rates. Clin Orthop Relat Res.
1988; 228: 79-87.