The Importance of a “Well-Rounded” Hip

This post comes from Fred Nelson, MD, an orthopaedic surgeon in the Department of Orthopedics at Henry Ford Hospital and a clinical associate professor at Wayne State Medical School. Some of Dr. Nelson’s tips go out weekly to more than 3,000 members of the Orthopaedic Research Society (ORS), and all are distributed to more than 30 orthopaedic residency programs. Those not sent to the ORS are periodically reposted in OrthoBuzz with the permission of Dr. Nelson.

Fifty years ago, the precise etiology of hip osteoarthritis (OA) was not clear. In 1976, Solomon proposed 3 potential causes of osteoarthritis in general:1

  1. Failure of essentially normal cartilage subjected to abnormal or incongruous loading for long periods
  2. Damaged or defective cartilage failing under normal conditions of loading
  3. Breakup of articular cartilage due to defective subchondral bone

In 1986, Harris expanded on this concept by noting that mild acetabular dysplasia and/or pistol grip deformity were associated with 90% of patients who had “so-called primary or idiopathic” hip OA.2 Harris further claimed that “when these abnormalities are taken in conjunction with the detection of other metabolic abnormalities that can lead to osteoarthritis of the hip,…it seems clear that either osteoarthritis of the hip does not exist at all as a primary disease entity or, if it does, is extraordinarily rare.”

Subsequently, acetabular dysplasia was defined as an acetabular shape where the lateral center edge angle (LCEA) was <25°, and the cam and pincer deformities were introduced as forms of acetabular dysplasia. Acetabular retroversion, as detected by the crossover sign seen in anterolateral hip radiographs, was recognized later, and Tonnis used CT imaging to determine acetabular and femoral anteversion.3

In 2020, investigators suspected that zonal-acetabular radius of curvature (ZARC) might play a role in hip-joint shape disorders.4 ZARC is the radius of curvature of the articular contact surface (from the margin of the fovea centralis to the acetabular rim), and the authors analyzed ZARC in anterior, superior, and posterior zones in subjects with normal, borderline, and dysplastic hips. (“Normal” was defined as LCEA of 25° to <40°; “borderline” as LCEA of 20° to <25°; and “dysplastic” as LCEA of <20°.) The 3-zone ZARC findings are summarized in the table below.

Mean Zonal-Acetabular Radius of Curvature (ZARC)

ZARC Zone Borderline Normal Dysplasia
Anterior 29.8 +/- 2.6 mm 28.0 +/- 2.2 mm 31.5 +/- 2.7 mm *
Superior 25.7 +/- 3.0 mm 25.9 +/- 2.2 mm 25.8 +/- 2.5 mm
Posterior 27.2 +/- 2.5 mm 26.4 +/- 1.9 mm 30.4 +/- 3.3 mm *

* P < 0.01

In this study, the severity of lateral undercoverage affected the anterior and/or posterior zonal-acetabular curvature. The take home message is that, absent metabolic abnormalities, acetabular and femoral head congruity and orientation are the driving forces in hip OA.


  1. Solomon L. Patterns of osteoarthritis of the hip. J Bone Joint Surg Br. 1976;58(2):176-83. Epub 1976/05/01. PubMed PMID: 932079.
  2. Harris WH. Etiology of osteoarthritis of the hip. Clinical orthopaedics and related research. 1986(213):20-33. Epub 1986/12/01. PubMed PMID: 3780093.
  3. Tonnis D, Heinecke A. Acetabular and femoral anteversion: relationship with osteoarthritis of the hip. J Bone Joint Surg Am. 1999;81(12):1747-70. Epub 1999/12/23. PubMed PMID: 10608388.
  4. Irie T, Espinoza Orias AA, Irie TY, Nho SJ, Takahashi D, Iwasaki N, et al. Three-dimensional hip joint congruity evaluation of the borderline dysplasia: Zonal-acetabular radius of curvature. J Orthop Res. 2020;38(10):2197-205. Epub 2020/02/20. doi: 10.1002/jor.24631. PubMed PMID: 32073168.

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