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CONTACT PATTERN AT THE GLENOHUMERAL JOINT AFTER REVERSED SHOULDER ARTHOPLASTY
Presentation O-172
Shoulder Biomechanics
S175
CONTACT PATTERN AT THE GLENOHUMERAL JOINT AFTER REVERSED SHOULDER ARTHOPLASTY Alexandre Terrier (1), Francesco Merlini (1), Alain Farron (2)
1. Laboratory of Biomechanical Orthopedics EPFL-HOSR, Switzerland; 2. Dpt of Orthopaedics and Traumatology, University of Lausanne, Switzerland
Introduction
Results
Reversed prostheses are being increasingly used as a satisfying solution for glenohumeral arthropathy associated with partial or severe rotator cuff deficiency [Boileau, 2006]. Despite reported complications, this non-anatomical implant provides indeed a significant improvement of pain relieve and mobility. The biomechanics of this implant is based on two main features: it is semiconstrained and it medializes its rotation centre. The constraint restores the lost stability, which was provided by the rotator cuff muscles, while the centre medialization increases the deltoid moment arm and thus the muscular strength, which was also diminished by the rotator cuff deficiency. Although the general mechanism of this prosthesis implant is known, the biomechanical gain is not clearly quantified. The aim of this study was thus to analyse the glenohumeral contact pressure of a reversed prosthesis during active abduction, and to compare to an anatomic prosthesis.
With the anatomic prosthesis, classical results were retrieved: joint contact force was maximal at approximately 90 degrees of abduction, reaching 86% of the body weight. With the reversed prosthesis, the contact force was 50% lower and the global muscular force was also approximately 50% less, but the overall deltoid force was only reduced by 20%. The moment arm of each deltoid part was up to 20 mm higher with the reversed design, particularly at the beginning of abduction. The centre of the glenohumeral contact area was also very different for each design. Classical results were retrieved with the anatomic design. With the reversed implant, the contact centre on the glenoid side (metal glenosphere) moved continuously from the inferior part towards its centre during abduction. Conversely, on the humeral side (polyethylene cup), the contact centre remained located in the same inferior position during the entire range of abduction. At 90 degrees of abduction, the maximum (resp. average) contact pressure was 1.2 PMa (resp. 0.5 PMa) with the reverse prosthesis, vs. 19.0 MPa (res. 7.9 MPa) with the anatomic one.
Methods The study was conducted by a 3D finite element model of the glenohumeral joint including 6 muscles: anterior, middle, and posterior deltoid, supraspinatus (SS), subscapularis and infraspinatus combined with teres minor. Glenohumeral joint stability was achieved by the compressive force of the muscles wrapping around the humerus, allowing for natural humeral head translation. Abduction was simulated in the scapular plane by a muscle activation algorithm [Terrier, 2007]. Scapulohumeral rhythm was set to 2:1. Arm weight was 37.5 N. A reversed prosthesis was compared to an anatomic one (Tornier, France). With the reversed implant, rotator cuff muscles were deactivated. Bone and metal parts were rigid, while polyethylene components were elastic. For both prostheses, the following quantities were calculated during the entire range of abduction: distribution of the contact pressure on the articular surface, maximum and average (contact force/contact area) contact pressure, equivalent contact force amplitude, direction and application point. Besides, the muscle forces and moment arms were also calculated.
16th ESB Congress, Oral Presentations, Tuesday 8 July 2008
Discussion The reverse prosthesis medializes the rotation centre, which increases the deltoid moment arms, and thus reduces the overall muscle force required for elevation. In this numerical study, this mechanism was reproduced. The contact force and contact pressure were quantified and compared to an anatomic prosthesis. The model confirmed that abduction with a reversed prosthesis can be performed without rotator cuff muscles, and even with a weaker deltoid, but produced very different contact pattern. Because of a lower contact force and a higher contact area, the glenohumeral contact pressure dropped considerably. This should be associated to a much lower lever of wear, assuming that impingements are avoided.
References Boileau P et al. J Shoulder Elbow Surg, 15:527540, 2006. Terrier et al. Clin Biomech, 22:645-651, 2007.
Journal of Biomechanics 41(S1)
Shoulder Biomechanics
S175
CONTACT PATTERN AT THE GLENOHUMERAL JOINT AFTER REVERSED SHOULDER ARTHOPLASTY Alexandre Terrier (1), Francesco Merlini (1), Alain Farron (2)
1. Laboratory of Biomechanical Orthopedics EPFL-HOSR, Switzerland; 2. Dpt of Orthopaedics and Traumatology, University of Lausanne, Switzerland
Introduction
Results
Reversed prostheses are being increasingly used as a satisfying solution for glenohumeral arthropathy associated with partial or severe rotator cuff deficiency [Boileau, 2006]. Despite reported complications, this non-anatomical implant provides indeed a significant improvement of pain relieve and mobility. The biomechanics of this implant is based on two main features: it is semiconstrained and it medializes its rotation centre. The constraint restores the lost stability, which was provided by the rotator cuff muscles, while the centre medialization increases the deltoid moment arm and thus the muscular strength, which was also diminished by the rotator cuff deficiency. Although the general mechanism of this prosthesis implant is known, the biomechanical gain is not clearly quantified. The aim of this study was thus to analyse the glenohumeral contact pressure of a reversed prosthesis during active abduction, and to compare to an anatomic prosthesis.
With the anatomic prosthesis, classical results were retrieved: joint contact force was maximal at approximately 90 degrees of abduction, reaching 86% of the body weight. With the reversed prosthesis, the contact force was 50% lower and the global muscular force was also approximately 50% less, but the overall deltoid force was only reduced by 20%. The moment arm of each deltoid part was up to 20 mm higher with the reversed design, particularly at the beginning of abduction. The centre of the glenohumeral contact area was also very different for each design. Classical results were retrieved with the anatomic design. With the reversed implant, the contact centre on the glenoid side (metal glenosphere) moved continuously from the inferior part towards its centre during abduction. Conversely, on the humeral side (polyethylene cup), the contact centre remained located in the same inferior position during the entire range of abduction. At 90 degrees of abduction, the maximum (resp. average) contact pressure was 1.2 PMa (resp. 0.5 PMa) with the reverse prosthesis, vs. 19.0 MPa (res. 7.9 MPa) with the anatomic one.
Methods The study was conducted by a 3D finite element model of the glenohumeral joint including 6 muscles: anterior, middle, and posterior deltoid, supraspinatus (SS), subscapularis and infraspinatus combined with teres minor. Glenohumeral joint stability was achieved by the compressive force of the muscles wrapping around the humerus, allowing for natural humeral head translation. Abduction was simulated in the scapular plane by a muscle activation algorithm [Terrier, 2007]. Scapulohumeral rhythm was set to 2:1. Arm weight was 37.5 N. A reversed prosthesis was compared to an anatomic one (Tornier, France). With the reversed implant, rotator cuff muscles were deactivated. Bone and metal parts were rigid, while polyethylene components were elastic. For both prostheses, the following quantities were calculated during the entire range of abduction: distribution of the contact pressure on the articular surface, maximum and average (contact force/contact area) contact pressure, equivalent contact force amplitude, direction and application point. Besides, the muscle forces and moment arms were also calculated.
16th ESB Congress, Oral Presentations, Tuesday 8 July 2008
Discussion The reverse prosthesis medializes the rotation centre, which increases the deltoid moment arms, and thus reduces the overall muscle force required for elevation. In this numerical study, this mechanism was reproduced. The contact force and contact pressure were quantified and compared to an anatomic prosthesis. The model confirmed that abduction with a reversed prosthesis can be performed without rotator cuff muscles, and even with a weaker deltoid, but produced very different contact pattern. Because of a lower contact force and a higher contact area, the glenohumeral contact pressure dropped considerably. This should be associated to a much lower lever of wear, assuming that impingements are avoided.
References Boileau P et al. J Shoulder Elbow Surg, 15:527540, 2006. Terrier et al. Clin Biomech, 22:645-651, 2007.
Journal of Biomechanics 41(S1)