Moment arms of the coracobrachialis and short head of biceps following a Latarjet procedure: a modeling study

BACKGROUND

The Latarjet procedure transfers the coracoid process to the anterior glenoid. This prevents recurrent anterior humeral dislocation but alters the origins of the coracobrachialis (CBR) and short head of the biceps (SHB). The impact of this alteration on the moment arms of these muscles has not been examined.

METHODS

The Newcastle Shoulder Model was updated with 15 healthy cadaveric bone models to create customized shoulder models. The CBR and SHB muscles were attached to the anterior glenoid via an elliptical wrapping object. Muscle moment arms were calculated for abduction, forward flexion, scapular plane elevation, and internal rotation with 20° and 90° of abduction. Statistical comparison of moment arms between native and Latarjet shoulders was performed using spm1D.

RESULTS

By transferring the origins of the CBR and SHB to the anterior glenoid, both muscles had extension moment arms during glenohumeral elevation in the coronal, sagittal, and scapular planes. Their average moment arms during abduction (-30.4 ± 3.2 mm for CBR and -29.8 ± 3.0 mm for SHB) and forward flexion (-26.0 ± 3.1 mm for CBR and -26.2 ± 3.2 mm for SHB) suggested that their role after the Latarjet procedure changed compared with their role in the native shoulder (P < .001). At higher abduction levels, both the muscles had higher internal rotation moment arms compared with the native shoulder.

CONCLUSION

The Latarjet procedure affected the moment arms of the CBR and SHB. Both muscles had increased extension and internal rotation moment arms at higher degrees of elevation compared with the native shoulders. This finding suggests that these muscles act as dynamic stabilizers after the Latarjet procedure.

Characterising moment arms of the coracobrachialis and short head of biceps for healthy and reverse total shoulder arthroplasty during elevation and rotation: a modelling study

BACKGROUND

Reverse total shoulder arthroplasty (RTSA) alters the line of action of muscles around the glenohumeral joint (GHJ). The effects of these changes have been well characterized for the deltoid but there is limited information regarding the biomechanical changes to the coracobrachialis (CBR) and short head of biceps (SHB). In this biomechanical study, we investigated the changes to the moment arms of CBR and SHB due to RTSA using a computational model of the shoulder.

METHODS

The Newcastle Shoulder Model (NSM), a pre-validated upper extremity musculoskeletal model was used for this study. The NSM was modified with bone geometries obtained from 3D reconstructions of 15 non-diseased shoulders, constituting the native shoulder group. The Delta XTEND prosthesis, with a glenosphere diameter of 38mm and polyethylene thickness of 6mm, was virtually implanted in all the models creating the RTSA group. Moment arms were measured using the tendon excursion method and muscle lengths were calculated as the distance between the muscles' origin and insertion points. These values were measured during 0°-150° of abduction, forward flexion, scapular plane elevation, and -90 to 60° of external-internal rotation with the arm at 20° and 90° of abduction. Statistical comparison between native and RTSA group was analyzed using spm1D.

RESULTS

Forward flexion moment arms had the greatest increase between RTSA (CBR:25.3±4.7 mm; SHB:24.7±4.5 mm) and native groups (CBR:9.6±5.2 mm; SHB:10.2±5.2 mm). CBR and SHB were longer in the RTSA group by a maximum value of 15% and 7% respectively. Both muscles had larger abduction moment arms (CBR: 20.9±4.3 mm; SHB: 21.9±4.3 mm) in RTSA compared to the native group (CBR: 19.6±6.6 mm; SHB: 20.0±5.7 mm). Abduction moment arms occurred at lower abduction angles in RTSA (CBR: 50º; SHB: 45º) compared to the native group (CBR: 90º; SHB: 85º). In the RTSA group, both muscles had elevation moment arms until 25° of scapular plane elevation motion, whereas in the native group the muscles only had depression moment arms. Both muscles had small rotational moment arms which were significantly different between RTSA and native shoulders during different ranges of motion.

CONCLUSION

Significant increases in RTSA elevation moment arms for CBR and SHB were observed. This increase was most pronounced during abduction and forward elevation motions. RTSA also increased the lengths of these muscles.

Development of a framework to assess the biomechanical impact of reverse shoulder arthroplasty placement modifications

Reverse shoulder arthroplasty biomechanics can be improved by modifying the placement of prosthesis. Biomechanical studies have quantified the impact of placement modifications on the mobility and stability of the reverse shoulder. While these studies have provided detailed insights, direct comparisons between their finding are obfuscated by their use of differing methodologies. The aim of our study was to develop an assessment framework which used musculoskeletal simulations to consistently evaluate the biomechanics of various placement modifications. We conducted musculoskeletal simulations of humeral elevations and rotations using 15 reverse shoulder models. For each model, these simulations were conducted for a reference configuration of the prosthesis, established using surgical guidelines, and 34 modified configurations, which were based on commonplace adaptations to the placement of the glenosphere and humeral tray. The effect of each modified configuration on deltoid elongation, deltoid moment arm (DMA), joint stability, and impingement-free range of motion (IFROM) was determined relative to the reference configuration. We found that 16 of the 34 modified placements had an overall beneficial impact on reverse shoulder biomechanics. Within this subset, we identified two biomechanical trade-offs. First, there is an antagonistic relationship between IFROM and both the DMA and joint stability. Second, functional requirements differ between humeral elevations and rotations. Furthermore, we found that posteromedial translation of the humeral tray had the most beneficial impact on joint stability and inferior translation of the glenosphere had the most beneficial impact on IFROM and DMA.

Effect of humeral tray placement on impingement-free range of motion and muscle moment arms in reverse shoulder arthroplasty

BACKGROUND

It has been suggested that onlay humeral tray placement in reverse shoulder arthroplasty can affect impingement and muscle functionality. This study investigates biomechanical changes to the reversed shoulder using a variety of tray positional configurations.

METHODS

The reconstructed scapula and humerus from 12 CT scans were used to customise a 3D biomechanical model of the shoulder. Each model underwent virtual RSA surgery using a commercially available prosthesis that was reconstructed from an explant. 17 tray positions were tested: the default location with no offset and 16 offset locations (2.5 and 5 mm radial offsets over 45° circumferential intervals). Impingement and muscle moment arms were measured during three standardised activities, and impingement was measured during an activity of daily living.

FINDINGS

Offset direction was found to have an effect (P < 0.05) on extra-articular impingement and muscle moment arms for all activities; whereas, offset distance did not (P > 0.05). Overall, impingement-free range of motion was maximised using a posterolateral tray offset and muscle moment arms were maximised using a medial tray offset. An antagonistic relationship between changes to impingement and muscle moment arms due to tray placement was identified and, consequently, the simultaneous maximisation of both outcome measures was not possible.

INTERPRETATION

The functional outcomes of reverse shoulder arthroplasty can be improved by altering onlay humeral tray placement. Due to the antagonistic relationship between the impingement and muscle moment arms, placement of the tray should be guided by patient-specific characteristics.