The reply by Dr. Argenson and S. Parratte that was originally posted here was withdrawn for corrections. The corrected letter is as follows (updated June 22, 2007):

J.-N.A. Argenson and S. Parratte reply:

We would like to thank Dr. Biedermann for addressing the problem of the controversial ideal value for cup positioning in total hip arthroplasty. Biedermann et al. pointed out that small variations from the desired 15° of anteversion and 45° of abduction can increase the frequency of dislocation and thus the cup placement should be in the optimal position ² . We concur with that statement; however, we must ask the question: What is the theoretical optimal cup position?

We agree with Biedermann et al. ² about the limitations of the Lewinnek "safe zone" ¹ used in our study; however, even in studies using accurate measurement techniques such as the EBRA (Ein Bild Roentgen Analyse) method or tridimensional computed tomography scan methods, there are actually no data concerning the optimal cup positioning angles that integrate individual pelvic tilt, individual bone stock, and cup/stem combined anteversion ^2-4 . Concerning the cup/stem combined anteversion, previous tridimensional anatomical studies have shown a large range of individual femoral anteversion angles, from 0.29° to 44.5° in patients with primary osteoarthritis and from 2° to 80° in those with developmental dysplasia of the hip ^5,6 . Concerning pelvic tilt, the values in our study ranged from −22° to 14° preoperatively, and Nishihara et al. reported that pelvic tilt values may change postoperatively, with a mean difference of 2° ± 7.5° (range, −26° to 15°) compared with the preoperative value ⁷ . According to DiGioia et al. ⁴ , the quoted figures for optimal acetabular alignment may not be applicable to every patient, and optimal alignment may need to be considered as a moving target rather than a fixed pair of abduction and anteversion angles.

For these reasons, when designing our study, we defined the "safe zone" as historically described by Lewinnek et al. ¹ as the "target value" for cup positioning with the free-hand and the computer-assisted method. We used 40° ± 10° of abduction and 15° ± 10° of anteversion as target values on the basis of the literature and considered these values to be more applicable than exact values of abduction and anteversion angles ^1,3 . The purpose of our study was to demonstrate that computer-assisted surgery can be an accurate tool with which to reach target values for cup angles, and we verified our hypothesis.

As mentioned in the Discussion of our paper, one of the next challenges in computer-assisted cup positioning will be the definition of the optimal cup position for each patient on the basis of accurate measurement methods and integrating individual values for pelvic tilt, patient bone stock, and cup/stem combined anteversion.

Jean-Noel A. Argenson, MD
Sebastien Parratte, MD
Corresponding author: Jean-Noel A. Argenson, MD, Service de Chirurgie Orthopédique, Hôpital Sainte-Marguerite, 270 Boulevard Sainte-Marguerite, 13009 Marseille, France, e-mail: jean-noel.argenson@ap-hm.fr

References

1. Lewinnek GE, Lewis JL, Tarr R, Compere CL, Zimmerman JR. Dislocations after total hip-replacement arthroplasties. J Bone Joint Surg Am. 1978;60:217-20.

2. Biedermann R, Tonin A, Krismer M, Rachbauer F, Eibl G, Stöckl B. Reducing the risk of dislocation after total hip arthroplasty. The effect of orientation of the acetabular component. J Bone Joint Surg Br. 2005;87:762-9.

3. Tannast M, Langlotz U, Siebenrock KA, Wiese M, Bernsmann K, Langlotz F. Anatomic referencing of cup orientation in total hip arthroplasty. Clin Orthop Relat Res. 2005;436:144-50.

4. DiGioia AM, Hafez MA, Jaramaz B, Levison TJ, Moody JE. Functional pelvic orientation measured from lateral standing and sitting radiographs. Clin Orthop Relat Res. 2006;453:272-6.

5. Husmann O, Rubin PJ, Leyvraz PF, de Roguin B, Argenson JN. Three-dimensional morphology of the proximal femur. J Arthroplasty. 1997;12:444 -50.

6. Argenson JN, Ryembault E, Flecher X, Brassart N, Parratte S, Aubaniac JM. Three-dimensional anatomy of the hip in osteoarthritis after developmental dysplasia. J Bone Joint Surg Br. 2005;87:1192-6.

7. Nishihara S, Sugano N, Nishii T, Ohzono K, Yoshikawa H. Measurements of pelvic flexion angle using three-dimensional computed tomography. Clin Orthop Relat Res. 2003;411:140-51.

To The Editor:

In their recent article on computer assisted cup postioning in THR (1), Parratte and Argenson found that 57% (seventeen) of thirty cups placed freehand and 20% (six) of thirty in the computer-assisted group were placed outside a so called safe zone (outliers)(P=0.002), as defined by Lewinnek et al. in 1978 (2) They concluded that the use of an imageless navigation system can improve cup positioning in total hip arthroplasty by reducing the percentage of outliers.

Lewinnek et al (2) proposed a radiographic safe range of the position of the cup as anteversion of 15°±10° and abduction of 40°±10°, although this was based on only nine dislocations.(Their rate of dislocation was 1.5% for cups placed within the so called safe zone,and it was 6.1% for outliers).

In contrast, in a more recent study of 137 dislocations, we (3) demonstrated that there is not a "safe" range for the position of the acetabular component although the lowest risk for dislocations was found in patients with cups placed in 15 degrees of anteversion and 45 degrees of abduction. Furthermore, there was a constant increase in the relative risk (odds ratio) of anterior dislocation as anteversion increased and vice versa for posterior dislocation. Patients with anteversion of less than 10° had a sixfold higher relative risk (odds ratio) for posterior dislocation than those with anteversion of 15°±5°. Patients with anteversion of more than 20° had a 6.3 times higher relative risk for anterior dislocation. In our study, 79% of all hips in the stable control group were positioned inside the safe zone as defined by Lewinnek et al. (1). However, the percentage of dislocated hips within the safe zone was significantly lower, but still 60% (Chi-Square test, p < 0.01)! Altering the safe zone to 45°±10° of abduction and 15°±10° of anteversion would include 93% of stable and even 67% of unstable hips (p < 0.01) (3).

Thus, cup placement must be in the optimal position. Any deviation from the optimal position will increase the risk of dislocation.

The author did not receive any outside funding or grants in support of his research for or preparation of this work. Neither he nor a member of his immediate family received payments or other benefits or a commitment or agreement to provide such benefits from a commercial entity. No commercial entity paid or directed, or agreed to pay or direct, any benefits to any research fund, foundation, division, center, clinical practice, or other charitable or nonprofit organization with which the author, or a member of his immediate family, is affiliated or associated .

References:

1. Sebastien Parratte and Jean-Noel A. Argenson Validation and Usefulness of a Computer-Assisted Cup-Positioning System in Total Hip Arthroplasty. A Prospective, Randomized, Controlled Study J Bone Joint Surg Am 2007; 89: 494-499

2. Lewinnek GE, Lewis JL, Tarr R, Compere CL, Zimmerman JR. Dislocations after total hip-replacement arthroplasties. J Bone Joint Surg [Am] 1978;60:217- 20.

3. Biedermann R, Tonin A, Krismer M, Rachbauer F, Eibl G, Stöckl B. Reducing the risk of dislocation after total hip arthroplasty. The Effect of Orientation of the Acetabular Component. R. J Bone Joint Surg [Br] 2005;87:762-9.