BIOMECHANICAL BEHAVIOUR OF MOBILE CUP SHOULDER PROSTHESES IMPLANTED FOR DEGENERATIVE SHOULDERS WITH A DESTROYED CUFF: RADIOCINEMATIC ANALYSIS OF 39 CASES

Purpose: The purpose of this work was to study the biomechanical properties of mobile cup shoulder prostheses and factors affecting their kinetics. Material and methods: Bipolar shoulder prostheses were implanted in 39 patients with degenerative shoulders and a destroyed cuff. Radiocinematic recordings of anterior elevation and active rotation were made at a mean 32 months follow-up (13 months–550 months). Results: Three types of biomechanical behaviour were observed for elevation movements. “Normal” behaviour was observed in 17 prostheses with preservation of the scapulohumeral rhythm and chronological participation of the three articular interfaces [intraprosthetic (head/cup), extra-prosthetic (cup/glenoid), scapulothoracic]. Mean anterior elevation was 114.7° for these shoulders. A “paradoxical” behaviour was observed in ten prostheses. Anterior elevation depended entirely on the scapulorthoracic joint, and was limited on the average to 42.5°. An “intermediate” behaviour was observed in 12 prostheses with inversion of the scapulohumearl rhythm. The glenohumeral mobility was decreased due to the absence of extraprosthetic mobility (eight cases) or intraprosthetic mobility (four cases). Mean anterior elevation in these shoulders was 80.83°. When the elevation behaviour was “normal”, the Constant score at last follow-up was significantly better compared with “intermediate” (p = 0.008) or “paradoxical” (p = 0.0001) behaviour. Three types of biomechanical behaviour were also observed for rotation movements: a “chronological “ behaviour was observed for 15 prostheses, via extraprosthetic mobility in all. Mean external rotation was 37.33° and mean internal rotation was 6.53 points. An “anarchic” behaviour was observed in 16 prostheses with a random proportion of intra- and extraprosthetic mobility. Mean external rotation was 8.75° and mean internal rotation was 4.25 points. For shoulders with “chronological” or “anarchic” behaviour, the mean external rotation (p = 0.002) and the mean internal rotation (0.04) were statistically better than shoulders with “truncated” behaviour. Discussion: An atrophic deltoid, mediocre joint congruency, and early-stage excentred scapular degeneration with preserved glenoid bone stock are factors favouring “paradoxical” elevation. Deltoid atrophy alone favours “truncated” rotation. This study was helpful in identifying conditions most appropriate for implanting this type of prosthesis and factors predicting postoperative outcome.