Clinically relevant differences in stress shielding between two short-stemmed femoral prostheses.

Background: Short-stemmed endoprostheses were developed to implement proximal load transmission and thus avoid stress-shielding in the proximal femur. Various prosthesis systems have been developed, which are discussed in the literature regarding stress shielding, clinical outcome, and long-term implant stability.

Methods: In this prospective randomized study, 52 patients (27 male, 25 female; average age 60.8 years) with conservatively unsuccessfully treated coxarthrosis were implanted with either a Nanos™ or Optimys™ short-stem prosthesis. Assessment included Gruen-zone based DEXA examinations immediately postoperatively and at one year to evaluate bone mineral density (BMD) and stress shielding, along with clinical outcomes using the Harris Hip Score (HHS). Radiographic measurements included offset (OFF), caput-collum-diaphyseal angle (CCD), leg length (LL), stem migration and inclination, and the occurrence of radiolucent lines (RL), assessed preoperatively, postoperatively, and at 12 months.

Results: DEXA showed differing stress-shielding profiles between stem types, favoring Optimys™ for BMD preservation. The Nanos™ group exhibited significantly greater BMD reduction in Gruen zones 1 (- 10.1%; p = 0.001), 4 (- 3.2%; p = 0.02), and 7 (- 21.3%; p = 0.001), whereas Optimys™ showed a significant decrease only in zone 7 (- 16.2%; p = 0.001). Although OFF, CCD, and LL changed significantly within groups postoperatively (p < 0.05), no statistically significant differences were found between the two stem designs in the final postoperative measurements (all p > 0.05). Stem migration remained clinically irrelevant in both groups. A statistically significant intra-group change was observed only in the Optimys™ group (Nanos™: 1.7 mm, p = 0.13; Optimys™: 2.5 mm, p = 0.01). Similarly, a small but statistically significant change in stem inclination was observed within both groups (Nanos™: 2.2°, p = 0.002; Optimys™: 1.5°, p = 0.01). Clinical improvement as measured by the Harris Hip Score (HHS) was excellent in both groups, with no significant differences between systems (Nanos™ pre/post: 52.0 / 98.0; Optimys™ pre/post: 51.6 / 97.0; both p < 0.001).

Conclusions: When compared, the Optimys stem demonstrated reduced stress shielding through improved proximal load transmission, resulting in significantly better preservation of bone mineral density in the proximal femur.

Level of evidence: Ib.