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Commentary and Perspective

How Prescient Can We Be?

Commentary on an article by Cody C. Wyles, MD, et. al.: “Creation of a Total Hip Arthroplasty Patient-Specific Dislocation Risk Calculator”

Blumenfeld, Thomas J. MD1

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The Journal of Bone and Joint Surgery: June 15, 2022 - Volume 104 - Issue 12 - p 1129
doi: 10.2106/JBJS.22.00384


Treating dislocation after primary or revision total hip arthroplasty is challenging. Over the last 22 years, the introduction of new or improved implants makes this complication seemingly technologically preventable. Dislocation secondary to the etiology of progressive liner wear and joint laxity leading to late dislocation has potentially been eliminated with the advent of highly cross-linked polyethylene liners1. Dislocation secondary to prosthetic or osseous impingement may be preventable with a combination of the use of larger femoral head sizes, the use of lateralized and/or anteversion-changing acetabular liners, and the appropriate use of an expanded array of femoral components2-4. Potential improvements in our ability to create intraoperative stability have been obtained through greater understanding of non-implant factors such as the interdependence of acetabular and femoral anteversion, or combined anteversion; intraoperative techniques to estimate combined anteversion (the Ranawat sign) or the use of computer navigation; continuing evaluation of the hip-spine relationship as first described by Lazennec et al. in 2004; and increased understanding of the preoperatively flexed or extended pelvis and the acetabular component positioning required to accommodate these different pelvic positions, combined with consideration of operative approach5,6.

While seemingly preventable, dislocation is likely unavoidable, and the salient issue for the surgeon is to try to reduce their patients’ dislocation rate. The understanding of what the patient literally brings to the table with regard to age, sex, body mass index (BMI), indication for surgery, and associated medical conditions is the first part of reducing the risk of dislocation. Preoperative planning to restore horizontal and vertical offset, and intraoperative execution of the same, combined with approach-specific trialing for impingement is clearly an art, not a defined science, and likely improves with years of practice7. Postoperative patient guidance on safe positioning of the hip remains valuable.

The important contribution of Wyles et al. highlights a potential path for improvement. In a study of 29,349 primary and revision total hip replacements performed at the Mayo Clinic from 1998 to 2018, the authors identified salient patient and technical (operative or implant) factors in patients who subsequently had a dislocation. Using these data, the authors then asked what factors were associated with instability. Each factor was weighted, and patient characteristics, operative approach, and operative technical choices (e.g., femoral head size and acetabular liner type) that were positively or negatively associated with dislocation were identified. From there, a clinical calculator was developed allowing the surgeon to describe a patient by age, BMI, diagnosis, and history of neurologic and/or spinal disease; identify the baseline dislocation risk; and examine operative choices and their effect on dislocation risk.

There are 2 caveats with regard to the applicability of the work. These caveats do not diminish the importance of the science shown. The first caveat is that, without radiographic parameters regarding restoration of vertical and horizontal offset and knowledge of the combined anteversion, a critical component of dislocation cannot be gleaned from this study. While the authors acknowledge this limitation, the question that is raised is important: if one has to choose between restoring offset and achieving a rational combined anteversion versus failing to do either or both while using a larger head, elevated liner, or different approach, will dislocation occur? The reader is cautioned that the surgeons involved in the study are highly experienced, and that perhaps the radiographic parameters were the same in the patients who did not have a dislocation. The balance between intraoperative execution and operative technical choices is not shown. The second caveat follows from the first. The exploration of the involved factors was based on those patients who subsequently had a dislocation. What is unknown is whether patients who did not have a dislocation differed from the group who did relative to the identified factors. If the groups differed, then that would suggest causation (e.g., that approach or liner type matter) as opposed to association.

Overall, Wyles and his Mayo Clinic colleagues contribute to our growing understanding of what surgeons may consider for each patient as we strive toward lowering the rate of dislocation. The take-home message may best be summarized as: choose the patient wisely, perform the surgery well, trial intraoperatively to determine if the options shown by the risk calculator apply to that specific patient, and vigilantly strive to improve.


1. Hopper RH Jr, Ho H, Sritulanondha S, Williams AC, Engh CA Jr. Otto Aufranc Award: Crosslinking reduces THA wear, osteolysis, and revision rates at 15-year followup compared with noncrosslinked polyethylene. Clin Orthop Relat Res. 2018 Feb;476(2):279-90.
2. Berry DJ, von Knoch M, Schleck CD, Harmsen WS. Effect of femoral head diameter and operative approach on risk of dislocation after primary total hip arthroplasty. J Bone Joint Surg Am. 2005 Nov;87(11):2456-63.
3. Kunze KN, Premkumar A, Bovonratwet P, Sculco PK. Acetabular component and liner selection for the prevention of dislocation after primary total hip arthroplasty. JBJS Rev. 2021 Dec 15;9(12).
4. Hoskins W, Bingham R, Hatton A, de Steiger RN. Standard, Large-Head, Dual-Mobility, or Constrained-Liner Revision Total Hip Arthroplasty for a Diagnosis of Dislocation: An Analysis of 1,275 Revision Total Hip Replacements. J Bone Joint Surg Am. 2020 Dec 2;102(23):2060-7.
5. Pour AE, Schwarzkopf R, Patel KP, Anjaria M, Lazennec JY, Dorr LD. Is combined anteversion equally affected by acetabular cup and femoral stem anteversion? J Arthroplasty. 2021 Jul;36(7):2393-401.
6. Lazennec JY, Charlot N, Gorin M, Roger B, Arafati N, Bissery A, Saillant G. Hip-spine relationship: a radio-anatomical study for optimization in acetabular cup positioning. Surg Radiol Anat. 2004 Apr;26(2):136-44.
7. Blumenfeld TJ. Pearls: clinical application of Ranawat’s sign. Clin Orthop Relat Res. 2017 Jul;475(7):1789-90.

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