'On-track' and 'off-track' shoulder lesions

Abstract

Shoulder stability depends on the position of the arm as well as activities of the muscles around the shoulder. The capsulo-ligamentous structures are the main stabilisers with the arm at the end-range of movement, whereas negative intra-articular pressure and concavity-compression effect are the main stabilisers with the arm in the mid-range of movement.There are two types of glenoid bone loss: fragment type and erosion type. A bone loss of the humeral head, known as a Hill-Sachs lesion (HSL), is a compression fracture of the humeral head caused by the anterior rim of the glenoid when the humeral head is dislocated anteriorly in front of the glenoid. Four out of five patients with anterior instability have both Hill-Sachs and glenoid bone lesions, which is called a 'bipolar lesion'.With the arm moving along the posterior end-range of movement, or with the arm in various degrees of abduction, maximum external rotation and maximum horizontal extension, the glenoid moves along the posterior articular margin of the humeral head. This contact zone of the glenoid with the humeral head is called the 'glenoid track'.A HSL, which stays on the glenoid track (on-track lesion), cannot engage with the glenoid and cannot cause dislocation. On the other hand, a HSL, which is out of the glenoid track (off-track lesion), has a risk of engagement and dislocation. Clinical validation studies show that the 'on-track/off-track' concept is able to predict reliably the risk of a HSL being engaged with the glenoid. For off-track lesions, either remplissage or Latarjet procedure is indicated, depending upon the glenoid defect size and the risk of recurrence. Cite this article: EFORT Open Rev 2017;2:343-351.

Keywords: Hill-Sachs lesion; glenoid bone loss; glenoid track; off-track lesion; on-track lesion; shoulder instability.

Fig. 1

Bony defects created anteroinferiorly. Anteroinferior bony defects were created at the 4:30 clock position stepwise.

Fig. 2

End-range stability: a) normal shoulder at the end-range of movement. The anterior capsule is tight; b) with a large glenoid bony defect (arrow), the end-range stability is still well preserved after the Bankart repair because the repaired capsule is tight and prevents the anterior translation of the humeral head.

Fig. 3

Mid-range stability: a) in the mid-range, the anterior capsule is lax and does not hold the humeral head in place; b) the head comes out of the glenoid socket because the socket is very shallow due to a large bony defect.

Fig. 4

Direction of dislocation. The head dislocates anteroinferiorly relative to the trunk, but anteriorly relative to the scapula.

Fig. 5

Bony defects created anteriorly. Anterior bony defects were created at 3:00 clock position stepwise.

Fig. 6

‘Best-fit’ circle method. A best-fit circle is applied, which fits perfectly to the posterior and to the inferior part of the glenoid. This circle is considered to be close to the original shape of the glenoid.

Fig. 7

Contralateral method: the contralateral shoulder (a), if it is intact, can be used as a control (dotted line) (b).

Fig. 8

Linear measurement: a) this is the contralateral glenoid, which is intact. ‘D’ represents the width of the intact glenoid; b) this is the involved side with a glenoid bony defect. ‘D’ comes from the intact side and the difference between the intact glenoid width ‘D’ and the width of the deficient glenoid is the defect width ‘d’. The size of the defect is expressed as d/D x 100 (%).

Fig. 9

Hill-Sachs lesion (HSL): arrows indicate the compression fracture of the posterior aspect of the humeral head, which is called a HSL. This lesion is located close to the greater tuberosity.

Fig. 10

Hill-Sachs lesion (HSL) located medially: this HSL is narrow, but located medially. This type of HSL has a high risk of becoming an ‘off-track lesion’.

Fig. 11

Hill-Sachs lesion (HSL) in the mid-range: it does not cause any instability in the mid-range of movement.

Fig. 12

Hill-Sachs lesion (HSL) entirely covered by the glenoid at the end-range: if the HSL is entirely covered by the glenoid when it comes to the end-range of movement, it is stable.

Fig. 13

Hill-Sachs lesion (HSL) not entirely covered by the glenoid at the end-range: a) the anterior rim of the glenoid is on the HSL; b) it engages with the HSL and a dislocation occurs.

Fig. 14

Hill-Sachs lesion (HSL) and the glenoid: a) this HSL is entirely covered by the glenoid at the end-range of movement. Therefore, this is a stable shoulder; b) the HSL is the same size as in (a), but it is not entirely covered by the glenoid due to a bony defect of the glenoid. Thus, this shoulder is unstable. The risk of the HSL engaging or not depends on the relative size of the HSL to the glenoid.

Fig. 15

Dynamic examination before the Bankart repair: a) the shoulder is unstable anteriorly because of the Bankart lesion; b) the head translates anteriorly during dynamic examination; c) as a result, the HSL easily engages and the head dislocates.

Fig. 16

Dynamic examination after the Bankart repair: a) The Hill-Sachs lesion (HSL) (the same size as in Figure 15) does not engage because the shoulder is stable due to the tight anterior capsule after the Bankart repair; b) this HSL is not covered by the glenoid after the Bankart repair; c) this may engage and the head may dislocate even after the Bankart repair. This is the true ‘engaging’ HSL.

Fig. 17

Glenoid track: when the arm is moved along the posterior end-range of movement keeping in maximum external rotation and maximum horizontal extension, the glenoid moves along the posterior articular surface of the humeral head. This contact zone is defined as the ‘glenoid track’. Reproduced with permission from Itoi E, Yamamoto N. Shoulder instability: treating bone loss. Current Orthop Practice 2012;23:609-615.

Fig. 18

Drawing of the glenoid track: a) on the ‘en face’ view of the intact glenoid, 83% of the glenoid width is obtained (0.83D); b) on the involved side, there is a defect (d; white dotted double-headed arrow). The width of the glenoid track is obtained by subtracting ‘d’ (black dotted double-headed arrow) from 83% value (0.83D - d; white double-headed arrow); c) this glenoid track width (0.83D - d) is applied to the posterior view of the humeral head. In this case, the HSL stays in the glenoid track, making this lesion an ‘on-track’ HSL.