The Cam Effect of the Proximal Humerus: Its Role in the Production of Relative Capsular Redundancy of the Shoulder
Bibliographic record
Abstract
PURPOSE: The purpose of this study was to investigate quantitatively the cam effect of the proximal humerus (whereby the anteroinferior capsule of the shoulder is tensioned in abduction-external rotation by virtue of asymmetric rotation of the eccentrically positioned humeral head), as well the extent to which a reduction in the cam effect would cause a relative capsular redundancy, in the cadaveric shoulder. METHODS: Five fresh-frozen cadaveric shoulders were tested. K-wires were used to sequentially position the shoulder in 3 different positions: neutral anatomic position, with contact at the glenoid bare spot; 90 degrees-90 degrees neutral shift position, with contact at the glenoid bare spot; and 90 degrees-90 degrees posterosuperior shift position, with contact 4.3 mm posterosuperior to the glenoid bare spot. The topographic anterior band distance (TABD) was measured with the shoulder in each of the 3 positions. The relative redundancy that occurred by shifting the glenohumeral contact point was determined by subtracting the TABD in the 90 degrees-90 degrees posterosuperior shift position from the TABD in the 90 degrees-90 degrees neutral shift position. RESULTS: The results of repeated-measures analysis of variance showed significant differences among the mean TABD values for the 3 positions (P < .010). There was a significant increase in TABD when the shoulder was brought from the neutral anatomic position to the 90 degrees-90 degrees neutral shift position (P < .020), confirming the cam effect of the proximal humerus in the 90 degrees-90 degrees position. Furthermore, the TABD decreased significantly when the 90 degrees-90 degrees neutral shift position changed to the 90 degrees-90 degrees posterosuperior shift position (P < .005), indicating a significant reduction in the cam effect with a concomitant relative redundancy in the anteroinferior capsuloligamentous complex. CONCLUSIONS: The proximal humerus produces a significant cam effect on the anteroinferior capsule when the shoulder is brought into a position of 90 degrees abduction and 90 degrees external rotation. A reduction in the cam effect as a result of a posterosuperior shift of the glenohumeral contact point with the shoulder in the 90 degrees-90 degrees position results in a relative redundancy of the anteroinferior capsuloligamentous complex. This relative capsular redundancy, coupled with the pseudolaxity that occurs with SLAP lesions, can produce a degree of apparent anterior laxity that is independent of any true translational anterior instability. CLINICAL RELEVANCE: The relative redundancy in the anteroinferior shoulder capsule caused by a decrease in the cam effect of the proximal humerus may have clinical implications in the pathophysiology of the disabled throwing shoulder. This relative redundancy is a secondary pseudolaxity, with the primary pathology being a tight posteroinferior capsule. Appropriate treatment (stretching of the posteroinferior capsule) of the primary pathology is a more appropriate initial treatment than instability surgery.
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.001 |
| Meta-epidemiology (narrow) | 0.001 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.001 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.003 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".