Static stabilisers of the glenohumeral joint in the dog - an in vitro study

by Gray, Michael.

To investigate the structural/anatomical components of the static stabilisers of the canine glenohumeral joint (GHJ). The GHJ of 22 complete cadaver specimens (44 limbs) were manipulated to estimate laxity at joint angles of 150º, 135º and 90º (palpation study). Following the removal of the peri-articular muscles, laxity was measured in 32 of these limbs after applying a 15N force to the scapula in a variety of directions whilst the humerus was fixed to a jig. This was repeated for the abovementioned angles in intact joints and after venting and flushing to eliminate the limited joint volume and adhesion/cohesion (LJV/AC) mechanisms (biomechanical study). Results of the palpation and biomechanical studies were compared. Anatomical dissections were also performed to better describe the structures constituting the static GHJ stabilisers. The palpation study revealed a marked variation between joints although there was a tendency for progressive laxity as the joint was flexed. This finding was supported by the biomechanical study which also demonstrated no significant difference between intact and vented joints except in a cranio-caudal direction at 135º and 90º, and in a medial/lateral direction at 135º. Static stabilisers contributed to the stability of the GHJ in extension but permitted an everincreasing range of motion as the joint was flexed. The results of the palpation study did not correlate well with the objective measurements of the biomechanical study. Manipulation is unlikely to be a reliable method for determining subtle joint laxity in the GHJ. The static stabilisers do not appear to play a significant role in providing joint constraint during normal range of motion (except in extension). Diagnoses of pathological laxity attributable to dysfunction of the static stabilisers should be made with caution and surgical treatments should take cognisance of the abovementioned biomechanical features. Anatomical structures providing constraint to joint mobility are described. University of Pretoria etd – Gray, M J (2005) 10 Terms and definitions There is some inconsistency in the veterinary literature with regard to the use of terms that describe glenohumeral joint (GHJ) pathology. For example, GHJ instability has been used to imply complete luxation in some studies, and subluxation in others. For the purposes of this study, the following definitions will apply: Translation: The movement of one joint surface relative to the other in a horizontal or vertical plane, when the one joint surface is fixed and a force is applied to the other. Luxation: Complete separation of the humeral head from the glenoid cavity such that the articulating surfaces are no longer in contact. Subluxation: Reduction in contact between articulating surfaces that is greater than the normal physiologic range of motion, but not to the point of complete separation. Instability: Instability incorporates both GHJ luxation and GHJ subluxation but implies that the luxation or subluxation is pathological and results in clinical symptoms and further joint damage. Laxity: Laxity refers to “slackness or looseness” of the joint. It may be physiologic or pathologic. In this study, the term is used to describe the magnitude of translation or rotation of the joint surfaces relative to one another when a force is applied. Primary stabiliser: A joint structure or mechanism that places a constraint on motion before any other. Such a stabiliser may be functional in one or many directions. Secondary stabiliser: A joint structure or mechanism that only places a constraint on motion once the primary stabilisers have been overcome. Note: In the human literature, the term “glenohumeral instability” has been used to differentiate patients that are symptomatic as a result of inappropriate joint laxity from those who are not (Matsen 1998). A supple athlete, for example, may have lax joints, but no symptoms. His or her joints would not be considered unstable. For a diagnosis of shoulder instability in people, the presence of pain, apprehension related to normal manoeuvres, crepitation, and lack of resistance to translation will all be evaluated and taken into consideration. In addition, diagnostic imaging techniques may be required. University of Pretoria etd – Gray, M J (2005) 11