The effect of humeral diaphyseal stem filling ratio on clinical and radiological outcome

Background

The stem of the reverse shoulder arthroplasty (RSA) used in this study population relies on diaphyseal anchorage. The surgical strategy was to insert the largest possible stem size to achieve an adequate press-fit fixation resulting in a high diaphyseal filling ratio (FR). However, for different reasons, a high FR was sometimes not achievable. The aim of this study was to analyze the effect of different FR (high/medium/low) on clinical and radiographic outcome (ie, subsidence and humeral peri-implant bone resorption), complications, and revision surgeries after RSA.

Methods

One hundred ninety nine patients were included and analyzed for clinical and radiologic outcome at a mean follow-up of 31.8 months after RSA. Diaphyseal humeral stem FR were measured directly postoperative and patients were divided into a low (N = 67), medium (N = 66), or high (N = 66) FR group. Humeral component complications and revision surgeries were measured as primary end points. Constant-Murley Score, range of motion, and the Subjective Shoulder Value were analyzed for clinical outcome. Periprosthetic bone resorption, radiolucent lines, shaft subsidence, and change in shaft alignment were measured for radiologic outcome.

Results

Two intraoperative fractures were seen in the high FR group (3%) and none in the other groups. There was no clinical humeral component loosening and no correlation could be found between FR and revision surgery. No differences were found between the low, medium, or high FR groups regarding range of motion, Subjective Shoulder Value, and absolute and relative Constant-Murley Scores (P > .05). Bone resorption was significantly more present in the high FR group, compared to the low FR ratio group, 4.5 (standard deviation ± 2.2) vs. 6.4 (standard deviation ± 1.8), respectively (P < .001). However, no difference could be observed regarding radiolucent lines, shaft subsidence, or shaft alignment changes between the 3 groups.

Conclusion

High humeral diaphyseal stem FR in RSA is associated with more humeral bone resorption and might increase the risk for intraoperative stem fractures, compared to medium and low humeral diaphyseal stem FR. However, the FR does not have an effect on functional outcome or radiographic loosening signs after a mid-term follow-up of 31.8 months.