Modular versus nonmodular neck femoral implants in primary total hip arthroplasty: which is better?

Background: Restoration of the hip center is considered important for a successful THA and requires achieving the right combination of offset, anteversion, and limb length. Modular femoral neck designs were introduced to make achieving this combination easier. No previous studies have compared these designs in primary THA, and there is increasing concern that modular designs may have a higher complication rate than their nonmodular counterparts.

Questions/purposes: We therefore asked (1) whether use of a stem with a modular neck would restore limb length and offset more accurately than a stem with a nonmodular neck, and (2) whether patients who received modular neck systems had better hip scores or a lower frequency of complications and reoperations than those receiving a comparable nonmodular stem.

Methods: Two cohorts of patients undergoing primary THAs, 284 patients with a nonmodular neck and 594 patients with a modular neck, were treated by one surgeon through a posterior approach. These were two nearly sequential series with little overlap. Harris hip scores and SF-12 outcomes surveys were administered at followup with a mean of 2.4 years (maximum, 5.9 years).

Results: In the modular neck cohort, a greater proportion of patients had equal (within 5 mm) radiographic limb lengths (89%, compared with 77% in nonmodular cohort p = 0.036), and a smaller offset difference (6.1 versus 7.5 mm, p = 0.047) was observed. Whether these statistical differences are clinically important is unclear. A smaller proportion of patients in the modular neck cohort achieved equal apparent or clinical limb length at 1 year (85% versus 95%, p < 0.001) and at 2 years (81% versus 94%, p < 0.001). In addition, these differences did not appear to result in better Harris hip or SF-12 scores, fewer complications, or reduced likelihood of revision surgery.

Conclusions: Use of modular neck stems did not improve hip scores nor reduce the likelihood of complications or reoperations. Because of their reported higher risks, there is no clear indication for modularity with a primary THA, unless the hip center cannot be achieved with a nonmodular stem, which is rare.

Level Of Evidence: Level III, therapeutic study. See the Instructions to Authors for a complete description of levels of evidence.