Quantitative cytochemical evidence for local increases in bone turnover at the acromial enthesis of the human coracoacromial ligament.

OBJECTIVE

Enthesophytic bone outgrowths are found at many ligament attachment sites, and while their incidence is associated with many pathologies, the mechanism by which they form remains controversial. We hypothesized that changes in local cell behavior, provoked by mechanical alterations within the coracoacromial ligament (CAL), lead to acromial enthesophyte formation. We investigated whether cell behavior at acromial entheses is consistent with this.

METHODS

We used quantitative enzyme cytochemistry to measure glucose 6-phosphate dehydrogenase (G6PD), alkaline phosphatase (ALP; osteoblastic activity), and tartrate-resistant acid phosphatase (TRAP; osteoclastic phenotype) activities in cells of the acromial attachment into the CAL in patients with rotator cuff tears.

RESULTS

(1) Resident osteoblasts on the acromion's inferior aspect express elevated activity of G6PD and ALP, indicative of increases in osteogenic potential. (2) These activities are selectively raised at the "leading edge" of acromial bone CAL enthesis. (3) In contrast, distribution of TRAP-positive cells does not exhibit a spatial correlation with enthesis architecture. We also found that cells situated close to the CAL attachment into the acromion exhibited elevated levels of G6PD and ALP activity, but intriguingly, also showed higher TRAP activity than neighboring cells distant from entheses.

CONCLUSION

These results suggest that the acromion in these patients undergoes bone accretion at the inferior attachment of the CAL, and that enthesial ligament cells close to the bone express characteristics consistent with enthesophyte formation at the leading edge of this bony spur's extension into the ligament.