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Acetabular Cup Positioning in Total Hip Replacement: The Impact of Obesity

Like the natural hip, the implant used in total hip replacement (THR) has a ball and socket design. The rounded socket area of the implant that fits in the pelvis is known as the acetabular cup. The correct placement of the acetabular cup during THR is an important determinant of implant stability, bearing surface wear and longevity.

Despite this importance, nationwide, the incidence of malpositioning has been reported to approach 50%. An established risk factor for incorrect positioning is having THR at a hospital that performs few such operations, or by a surgeon with low experience.

What has also been considered a significant risk factor is patient obesity. However, a recent HSS study shows this may not be the case.

HSS performs more hip surgeries than any other hospital, so neither low surgical volume nor surgeon experience are factors for the hospital. However, at HSS, and nationwide, patient obesity continues to increase.

To determine how significant a factor patient obesity can have on acetabular cup positioning, HSS recently compared THR outcomes in 120 obese patients to THR outcomes for 120 non-obese patients.

Results showed, that at HSS, for an experienced high-volume surgeon using the posterolateral approach, obesity did not increase malpositioning of acetabular cups in primary THR.

Two Weight Groups Compared

The study compared two groups of patients who received THR at HSS by the same highly experienced surgeon.

  • One group was a consecutive series of 120 non-obese patients.
  • The other group was a consecutive series of 120 obese patients.

In the study, patients were classified as obese if their body mass index (BMI) was greater that 30. The obese group had a mean BMI of 34.4, with their BMI ranging between 30 and 49.8. The non-obese group had a mean BMI of 25.4, ranging from 14.5 - 29.7.

Diagnostic imaging was performed before surgery and at the first follow-up visit 6 weeks after surgery, when detailed pelvic images were taken to analyze the acetabular cup positioning. Those specific x-ray images are known as AP pelvis and cross-table radiographs. Using the images, measurements were taken to assess cup orientation and angle of inclination for each patient. Then the measurements taken of each group were statistically compared.

Weight Not a Factor

Analysis showed that there was no significant association between BMI and cup position.

Both groups showed excellent cup position and results. The comparison showed that increased weight did not result in increased malposition of the acetabular cups.

Previous research has well established that both the surgeon’s experience in THR, as well as the hospital’s volume of THR procedures, impacts the outcomes of hip replacement. This new study indicates that surgeon’s experience counts when it comes to obese patients. Experience with a broad range of patients with hips in all shapes and sizes favorably affect outcome. The study does show when THR is performed by a highly experienced surgeon at a high volume hospital like HSS, obesity is not a risk factor for poor acetabular cup position.


Benjamin A. McArthur, MD
Hospital for Special Surgery

Ettore Vulcano, MD
Hospital for Special Surgery

Denis Nam, MD
Hospital for Special Surgery

Joseph Nguyen, MPH
Hospital for Special Surgery

Headshot of Eduardo A. Salvati, MD
Eduardo A. Salvati, MD
Attending Orthopaedic Surgeon, Hospital for Special Surgery
Clinical Professor of Surgery (Orthopaedics), Hospital for Special Surgery

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