Harryman et al. used 6-degrees-of-freedom motion tracking and a 6-axis force transducer on 7 cadaveric glenohumeral joints to quantify humeral head translation during passive motion. The study tested whether translation is driven by capsular tension and whether it changes when the posterior capsule is selectively tightened. This is the foundational cadaveric study establishing that glenohumeral translation is a normal, capsule-driven phenomenon — not a sign of instability.
The shoulder does not function as a pure ball-and-socket joint. Humeral head translation is a normal, predictable part of glenohumeral kinematics — but it becomes pathologic when capsular asymmetry amplifies it.
The capsular constraint mechanism is the key concept: a tight posterior capsule acts as a fulcrum, obligately driving the humeral head anteriorly and superiorly during flexion and cross-body movement. This is the mechanistic explanation for why posterior capsular contracture (limited internal rotation, painful cross-body reach) produces anterior impingement symptoms rather than posterior ones.
When you see a patient with impingement and restricted internal rotation or cross-body reach, think posterior capsular tightness first. The correct treatment targets the tight posterior capsule (sleeper stretches, posterior capsular release). Not the anterior structures.
This paper is also why overly tight anterior repairs (e.g., for instability) can cause posterior humeral head migration: the same mechanism works in reverse, and excessive anterior tightening can drive the head posteriorly into glenohumeral arthritis.
Harryman et al. used 6-degrees-of-freedom motion tracking and a 6-axis force transducer on 7 cadaveric glenohumeral joints to quantify humeral head translation during passive motion. The study tested whether translation is driven by capsular tension and whether it changes when the posterior capsule is selectively tightened. This is the foundational cadaveric study establishing that glenohumeral translation is a normal, capsule-driven phenomenon — not a sign of instability.
The shoulder does not function as a pure ball-and-socket joint. Humeral head translation is a normal, predictable part of glenohumeral kinematics — but it becomes pathologic when capsular asymmetry amplifies it.
The capsular constraint mechanism is the key concept: a tight posterior capsule acts as a fulcrum, obligately driving the humeral head anteriorly and superiorly during flexion and cross-body movement. This is the mechanistic explanation for why posterior capsular contracture (limited internal rotation, painful cross-body reach) produces anterior impingement symptoms rather than posterior ones.
When you see a patient with impingement and restricted internal rotation or cross-body reach, think posterior capsular tightness first. The correct treatment targets the tight posterior capsule (sleeper stretches, posterior capsular release). Not the anterior structures.
This paper is also why overly tight anterior repairs (e.g., for instability) can cause posterior humeral head migration: the same mechanism works in reverse, and excessive anterior tightening can drive the head posteriorly into glenohumeral arthritis.