While posterior decentering of the humeral head on the glenoid may be noted on a preoperative axillary “truth view” taken with the arm in a position of functional elevation
it is not always evident before shoulder arthroplasty.
At surgery, after osteophyte resection and anterior releases, posterior instability may first appear during trialing. In the series of Hsu et al, 11 of the 33 shoulders that needed an anteriorly eccentric head had less than 5% posterior decentering before surgery, and decentering did not correlate with glenoid version. Leaving decentering unaddressed can lead to inferior outcomes [3].
Correcting glenoid version does not always settle the matter. After anterior asymmetric reaming of 62 B2, B3, or C glenoids, posterior subluxation had returned in 25 to 41 of them at a mean of 8.3 years, depending on how it was measured; glenoid loosening was more frequent when it persisted, although clinical scores were not worse [14]. Gerber et al, by contrast, found static subluxation reversed in 21 of 23 shoulders at a mean of 42 months [13]. Chamberlain et al reamed B2 glenoids to within about 10° to 15° of normal version and still found excessive posterior subluxation with trials in place in 32 of 114 shoulders [4]. The practical questions are how to recognize excess posterior laxity with trials in place, and what can be done about it on the humeral side.
Consider a very active 40-year-old man who did not want plastic in his shoulder and chose a ream and run.
Osteoarthritis
Posterior decentering on axillary “truth” view, minimal retroversion
Preoperative plan suggested a 50-20 head
Under anesthesia, the shoulder was stiff
The glenoid was biconcave
And was conservatively reamed to a monoconcavity
Trialing with the planned 50-20 head revealed excessive soft tissue tightness. Trialing with a 50-18 concentric head revealed excessive posterior translation. Trialing with an anteriorly eccentric head revealed excellent balance of mobility and stability.
Motion at the end of the case.
Assisted flexion started on day 1. He returned to his favorite activities.
Our approach, step by step
1. The intraoperative check
When to test
Once the osteophytes are removed, the soft tissues released, and the glenoid prepared, the shoulder is a different joint than it was before surgery. With the trial humeral component in place, the surgeon can evaluate its mobility and stability [4].
What to test
The 40/50/60 parameters are a useful guide: 40° of external rotation with the subscapularis approximated to its insertion on the lesser tuberosity, 50% posterior translation of the head on the glenoid under posteriorly directed pressure, and 60° of internal rotation with the arm abducted 90° [1]. More translation suggests too much laxity; failure to reach 40° of external rotation, 60° of internal rotation, or 150° of flexion suggests too much tightness.
The second test is posterior drop-back, or “shake and bake”: with trials in place, elevate the arm and give it a gentle shake. If the head drops back out of the glenoid, stability may need to be added [1, 2]. Clinically, posterior decentering is usually a problem when the arm is elevated; a shoulder that looks centered with the arm at the side may decenter during functional elevation [3, 11].
Thresholds used in published series
Published series have used different arm positions and thresholds for excessive posterior translation.
Source | Arm position and maneuver | Threshold for action |
Matsen & Lippitt [1] (40/50/60 rule) | External rotation with subscapularis approximated; posterior translation of the head on the glenoid; internal rotation with the arm abducted 90° | Reconstruction should allow 40° ER, 50% posterior translation, and 60° IR |
Matsen & Lippitt [1]; Clinton et al [2] | Forward elevation of the arm with trials in place | Head drops back out of the glenoid on elevation |
Hsu et al [3] | Passive elevation of the arm | More than 50% posterior subluxation of the humeral head |
Chamberlain et al [4] | Arm resting in adduction and neutral rotation; posterior translation | Head translates more than 100% posteriorly and/or rests more than 50% subluxated on the glenoid |
Szolomayer et al [15] (posterior capsular plication) | Anterior–posterior translation in neutral rotation with trials in place, after sizing components | Translation greater than 50% with trial components sized preoperatively |
2. Balancing mobility and stability on the humeral side
The steps run from least to most consequential. Repeat the Section 1 checks after each.
2.1 Confirm head size and height
First confirm that the initial trial head matches the resected head in diameter and thickness and that the neck cut is appropriate. In cadavers, head size and articular conformity each affected laxity and motion [7].
In a shoulder that was very stiff before surgery, the surgeon may deliberately choose a thinner head for motion rather than to reproduce the anatomy [17]. If this “understuffing” allows excessive posterior translation, the steps below are usually sufficient to restore stability.
2.2 If the trial concentric head translates excessively
consider replacing it with a head that is anteriorly eccentric.
The articular surface then sits centered in the glenoid while the humeral shaft and tuberosities remain in their posterior position [1, 4, 5].
The ream and run. As shown in the case presented above, the eccentric head is often needed in the young, active man who chooses a ream and run. Preoperative radiographs not infrequently show posterior decentering,
and without a polyethylene glenoid component, nothing on the glenoid side adds stability. Here the anteriorly eccentric head is particularly useful [3].
After a ream and run with an anteriorly eccentric head, the prosthetic head is centered on the glenoid.
A few more examples; there are many more.
Laboratory evidence. In 14 cadaveric total shoulders with 10° or 20° of glenoid retroversion created by eccentric reaming, Kim et al tested each head in the anatomic and the anterior offset position. At 20°, anterior offset increased the force needed to translate the head 10 mm posteriorly by 64% and the energy by 75%, and shifted contact anteriorly. At 10°, force increased by 26% and the contact changes were not significant [5].
A finite element model found a modest anterior shift in the center of pressure (about 1.4 mm) and a more anteromedial muscle force vector [6].
Clinical evidence. Hsu et al used a 4-mm anteriorly eccentric head when posterior subluxation exceeded 50% on passive elevation in 33 shoulders: 24 ream and runs and 9 total shoulders, 16 with a rotator interval plication [3]. Decentering on functional axillary views fell from 10.4% to 0.9%, and the Simple Shoulder Test improved from 4.8 to 10.0 at a minimum of two years. None was revised for instability; two were revised for pain and stiffness. Without any deliberate attempt to change it, retroversion fell from 19.8° to 15.5° (P = .001) [3].
Chamberlain et al used the technique in 20 total shoulders with B2 glenoids (mean retroversion 31° in the 15 with CT), with partial version correction by anterior reaming, a pegged all-polyethylene glenoid, and no plication [4]. At about four years, the SST improved from 4.9 to 9.8 and the ASES score from 32.7 to 86.2; 18 of 20 reached the MCID for the SST and 19 of 20 for the ASES [4, 12]. Decentering fell from 9.9% to 0.5%, with none more than 5% posterior. No glenoid was loose at 24 months, and there were no revisions or instability.
The goal is a centered head, not an anteriorly displaced one: centering was maintained within 5% in all but one of the shoulders of Hsu et al [3] and in all of those of Chamberlain et al [4].
Technical points. Keep the head height correct relative to the greater tuberosity while inserting the offset component [4]. The shift is limited by the offset built into the head, 2 to 4 mm, and is often less than the maximum [3, 5]. The head overhangs anteriorly and may leave posterior cut surface uncovered; Chamberlain et al left that bone when it did not engage the glenoid in 40° of external rotation [4]. The subscapularis must drape over the overhang, which lifted it in cadavers [5]. Every lesser tuberosity osteotomy in the series of Chamberlain et al healed [4], and Hsu et al found no clinical subscapularis failure after peeling it [3].
2.3 Add a rotator interval plication when the eccentric head alone is not enough
If posterior translation is still excessive with the definitive humeral component in place, plicate the rotator interval [1]. In cadavers, imbricating the interval reduced posterior and inferior translation, and external rotation [8]; Hsu et al added it in about half their shoulders [3]. Recheck that 40° of external rotation remains [1].
2.4 Posterior capsular plication
Posterior capsular plication tightens the redundant posterior capsule to hold the head forward, rather than accommodating the posterior humerus. Kim et al describe its efficacy as questionable [5], and at revision for posterior instability it has often failed [16].
Szolomayer et al plicated 19 of 138 total shoulders with one or two purse-string stitches when translation in neutral rotation exceeded 50% with anatomically sized components. Seven of the 14 with a preoperative axillary view showed posterior subluxation [15].
At a mean of 8.9 months, forward elevation improved from 91° to 131° and external rotation from 15° to 51°. One patient needed an arthroscopic release for adhesive capsulitis, and none of the 12 contacted at a mean of 45 months reported dislocation, loss of motion, or further surgery [15].
2.5 A thicker head is a last resort
A thicker head reduces laxity by overstuffing, and in cadavers larger heads reduced motion as well [7]. It is an option for drop-back [1], but it trades instability for stiffness and higher joint loads.
2.6 Changing humeral version is not supported
With the glenoid component in 15° of retroversion, anteverting the humeral component by 15° did not increase resistance to posterior translation [9], and in another cadaver study humeral rotation did not compensate for glenoid version [10]. Re-cutting or re-cementing the humerus adds morbidity for a benefit that has not been demonstrated.
2.7 When humeral-side steps fail
If the head still drops back after sizing, anterior offset, and plication, look to the glenoid for a residual posterior facet or incomplete concave reaming in a ream and run, and reconsider whether the posterior cuff is competent. If instability remains, a glenoid component may need to be considered.
2.8 Confirming the result
Document centering on a standardized axillary view with the arm elevated in the plane of the scapula, where decentering shows itself [3, 11]; a view with the arm at the side may look centered. Chamberlain et al did not state their arm position, so their values may not compare directly with those of Hsu et al [3, 4].
Demonstrated and inferred
These reports are uncontrolled case series with short-to-intermediate follow-up [4]. No study has compared the eccentric head with version correction, augmented glenoids, or reverse arthroplasty in shoulders with the same intraoperative decentering.
Demonstrated: in cadavers with 20° of glenoid retroversion, anterior offset of the head increases resistance to posterior translation, with smaller effects at 10° [5], while anteverting the humeral component does not [9]; two case series report recentering on functional axillary views with SST gains well above the MCID [3, 4, 12]; posterior subluxation often returns after asymmetric reaming alone [14]; and posterior capsular plication at revision fails often [16].
Inferred: that the intraoperative thresholds in the table identify the shoulders that need treatment; that the eccentric head, rather than the accompanying glenoid preparation, plication, or the rest of the operation, produces the recentering; and that recentering itself produces the improvement in function.
Conclusions from our practice
1. Trial intraoperatively: mobility and stability cannot be reliably predicted before surgery.
2. Posterior instability is a particular risk in the ream and run, which has no glenoid component to add stability; much of our experience with the anteriorly eccentric head comes from this procedure.
3. If a concentric trial head translates excessively, consider an anteriorly eccentric head.
4. If posterior instability persists, consider a rotator interval plication.
5. With this approach, residual posterior instability, glenoid loosening, and subscapularis failure have been rare, and patient-reported outcomes have consistently exceeded the MCID.
6. Because our patients have done well with this method, we have not conducted randomized comparisons with other approaches.
It’s about balance
Black necked Stilt
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References
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2. Clinton J, Warme WJ, Lynch JR, Lippitt SB, Matsen FA 3rd. Shoulder hemiarthroplasty with nonprosthetic glenoid arthroplasty: the ream and run. Tech Shoulder Elbow Surg. 2009;10(1):43-52.
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