The aim of this studdy was to examine the effect of cementless surface replacement shoulder arthroplasty (CSRA) on proximal humeral anatomy in eroded shoulder joints.

AP radiographs of 39 shoulders in 37 patients that underwent CSRA for arthritis were examined for geometry of the glenohumeral joint with correction for the magnification of our apparatus. Thirty-two were hemiarthroplasties and seven were total shoulder replacements (TSR). Average age was 70 years (range: 29–88 years). Mean clinical and radiological follow-up was 38 month (range: 24–72 months) and 16 months (range: 10–65 months) respectively. We measured reliable values on the proximal humerus and the lateral glenohumeral offset (LGHO) relative to the coracoid base. Preoperative and last follow-up Constant scores were recorded.

Based on anatomical data with respect to humeral head radius there was a mean 6mm preoperative loss in LGHO (95% CI 3.6–8.8, p< 0.01). The mean value of 53mm increased postoperatively to 59mm and was 57mm at last follow-up. The mean changes were an increase of 6mm (95% CI 4.4–8.5, p< 0.01) and then a decrease of 2mm (95% CI 0.1–5.4, p=0.04) respectively. The lever arm, measured from the greater tuberosity to the centre of instant rotation, increased a mean 5mm post-operatively (95% CI 3.8–6.4, p< 0.01) with no significant fall at last follow-up. Humeral head size and medial offset relative to shaft width increased by 13% and 30% respectively. The humeral head centre moved superiorly relative to the glenoid a mean 2mm after operation (95% CI −0.2–3.5, p=0.08) and a further 1mm at last follow-up (95% CI −0.1–3.0, p=0.07). Forward flexion and abduction improved from 66′ and 58′ preoperatively to 124′ and 112′ postoperatively, with age/ sex-adjusted Constant scores increasing by 53 (95% CI 43.0–64.4, p< 0.01) from a mean 25 preoperatively to 79 at last follow-up.

For hemiarthroplasty the LGHO increased by 9% and for TSR by 24%, with greater increases in flexion and abduction in the latter group.

The Copeland CSRA is centred on the native humeral neck for head version and offset. This preserves maximal bone stock and avoids the need for modularity which some modern stemmed prostheses use to reconcile differences between proximal shaft and humeral head anatomy. The inherent limitation is the requirement for preservation of sufficient humeral head to permit resurfacing. In this group with fairly marked degrees of joint erosion the CSRA, using autogenous bone graft and prostheses of variable width, achieved statistically and clinically significant increases in the lever arm. The improved biomechanics and soft tissue tension correlated to a good clinical outcome with no evidence of significant early subsidence.