Terrier (2008) — Simulated joint and muscle forces in reversed and anatomic shoulder prostheses

Simulated Joint and Muscle Forces in Reversed and Anatomic Shoulder Prostheses

·The Journal of Bone and Joint Surgery. British volume·2008·127 citations·Shoulder & Elbow

This finite element study compares a reverse and an anatomic shoulder prosthesis during simulated scapular-plane abduction from 0° to 150°. It quantifies how the reverse design compensates for a deficient rotator cuff by measuring joint force, deltoid force, and deltoid moment arm.

Study Snapshot

Design

Finite element model

Setting: Biomechanics laboratory, single institution

Funding: Industry (Tornier)

Objective

Whether a reverse prosthesis reduces joint and deltoid forces versus an anatomic implant during abduction with cuff deficiency

Outcome(s)

Glenohumeral joint force, deltoid force and moment arm across 0-150° abduction

Subjects

One reconstructed cadaver shoulder

Anatomic Aequalis prosthesis
Reverse Aequalis, complete cuff deficiency
Reverse Aequalis, supraspinatus-only deficiency

Key Findings

The reverse prosthesis halved glenohumeral joint force when the cuff was absent. Peak force was 313 N (42% body weight) for the reverse vs 648 N (86% body weight) for the anatomic implant. This lower, more congruent loading should reduce polyethylene wear.
Medialising the center of rotation roughly doubled the deltoid moment arm, increasing it by about 20 mm, most effectively at the start of abduction. This is why the reverse design improves movement initiation specifically.
Abduction was possible without any rotator cuff muscle, but still required 80% of the deltoid force used with the anatomic prosthesis. So the deltoid does the work the cuff no longer can, and only saves about 20%.
Total muscle force needed for abduction dropped by a factor of 2 (700 N anatomic vs 320 N reverse). This reflects both the longer lever arm and the absent cuff force to counterbalance.
With supraspinatus-only deficiency, benefits were smaller. Joint force fell 30% (465 N, 62% body weight) and 88% of deltoid force was still required, but the contact force was more centred.
No rocking-horse effect appeared on the humeral component, unlike the glenoid displacement seen with anatomic implants. The authors attribute humeral failure to poor bone support or osteolysis rather than mechanics.

Board PearlMedialising the center of rotation doubles the deltoid moment arm and halves joint force — this is why the reverse prosthesis works without a rotator cuff.

Clinical Relevance

Understand the reverse prosthesis as a lever-arm solution: medialising the center of rotation doubles the deltoid moment arm, letting the deltoid abduct the arm without a rotator cuff. That single mechanical idea explains why you select this implant for cuff-deficient arthropathy and why an intact, functioning deltoid is a prerequisite.

The 50% drop in joint force is the durability argument: lower, more vertically aligned contact means less polyethylene wear and no glenoid rocking-horse effect. Clinically, protect the anterior deltoid, especially in revision cases, since the model shows it carries a large share of the abduction load.

Remember the limits of this evidence: this is a finite element model of one cadaver shoulder simulating only scapular-plane abduction, so it explains mechanism rather than proving clinical outcomes.

Supplemental

Simulated Joint and Muscle Forces in Reversed and Anatomic Shoulder Prostheses

Terrier, Farron, et al.·The Journal of Bone and Joint Surgery. British volume·2008·127 citations·Shoulder & Elbow

Simulated Joint and Muscle Forces in Reversed and Anatomic Shoulder Prostheses

Study Snapshot

Key Findings

Clinical Relevance

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Simulated Joint and Muscle Forces in Reversed and Anatomic Shoulder Prostheses

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Simulated Joint and Muscle Forces in Reversed and Anatomic Shoulder Prostheses

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Simulated Joint and Muscle Forces in Reversed and Anatomic Shoulder Prostheses

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