AMERICAN SOCIETY OF BIOMECHANICS
Presented at the Twenty-First Annual Meeting
of the American Society of Biomechanics
Clemson University, South Carolina
September 24-27, 1997
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| AMERICAN SOCIETY OF BIOMECHANICS
Presented at the Twenty-First Annual Meeting |
To determine the optimal treatment of patients with facioscapulohumeral muscular dystrophy, three different techniques of scapulothoracic fusion were compared using a novel cadaveric model. Ribs were removed from cadavers and mounted in dental stone in anatomic position. Scapulae were then fused to the ribs using single wire fixation, double wire fixation or tension band wiring as previously descibed. Constructs were loaded to failure and biomechanical properties of the three techniques were compared.
Facioscapulohumeral (FSH) Dystrophy is an uncommon form of muscular dystrophy primarily involving the muscles of the face and shoulder stabilizers. First described by Duchenne in 18621,this disease has an autosomal dominant inheritance and is usually diagnosed in the third decade of life. Although longevity is rarely affected, patients are severely limited in forward flexion and abduction of the arm. In the subset of patients whose deltoid and supraspinatus function is spared, surgical fixation of the scapula to the rib cage has been shown to restore useful range of motion, enhance physical appearance, improve exercise tolerance and activities of daily living. A variety of methods of scapulothoracic fusion have been described including single wire fixation and double wire fixation2, tension band wiring3, fascial tethering4, plate and screw fixation5, and tibial strut bone grafting6. However, to date there has been no direct comparison of the biomechanical properties of these methods of scapulothoracic fusion. The purpose of the present study was to compare the biomechanical properties of three described techniques of wire fixation in a cadaver model.
Both scapulae and the fourth, fifth, and sixth pairs ribs were harvested from ten adult cadavers. The specimens were debrided free of soft tissue, cleaned, and mounted in dental stone in anatomic position. Scapulae were then fused to the ribs using single wire fixation (n=5), double wire fixation (n=8), or tension band wiring (n=6) with 18 gauge stainless steel wire. The samples previously had been randomly assigned to one of the three groups. Following the fixation procedures, samples were carefully inspected for fractures of either the ribs or scapulae and to ensure no laxity existed at the stone-rib interface. Constructs were then mounted on an Instron materials testing system using c-arm clamps such that the piston of the MTS would contact the base of the glenoid process and exert a downward or inferiorly directed force upon the scapula. The constructs were subsequently loaded in distraction to failure. Load-deformation curves were generated using a chart recorder linked to the load cell of the system. The following three parameters were examined: maximum load to failure (kg), stiffness (kg/mm), and displacement to failure (mm). Maximum load to failure values were determined by measuring the highest force in kilograms tolerated by the construct prior to failure. Failure was arbitrarily defined as the first major fracture sustained by the construct. Stiffness was calculated using the slope of the initial portion of the load-deformation curve. Finally, displacement to failure was defined as the distance in millimeters traveled by the MTS piston prior to failure. The mean values of the three groups were compared using a t-test with p < 0.05 required for statistical significance.
There were no significant differences in fracture pattern among the three methods of fixation. A summary of the three study parameter results is given in Figure 1.
Figure 1: Results
The double wire fixation group had a significantly higher maximum force (19.8kg) than the single wire fixation group (10.4kg, p < 0.01). The tension band group had a significantly higher maximum force (25.1kg) than both the single wire group (p < 0.001) and the double wire group (p < 0.05). There were no significant differences among the three groups with respect to the stiffness parameter measured. The tension band group (16.8mm) had a significantly higher displacement to failure than both the single wire group (8.5mm, p < 0.05) and the double wire group (7.1mm, p < 0.01).
Failure of fixation and rib and scapula fractures have been reported as complications following scapulothoracic fusion often requiring revision surgery. In the present experiment, fractures occurred at significantly higher forces using the tension band technique. This suggests that mechanical failure would be less likely using this method of fixation in vivo. Constructs can be loaded to failure in only one direction; an inferior distraction force was chosen since the weight of the patient's arm would represent the most significant force acting on the construct. Immobilization of the ribs reduces compliance of the chest, thus functionally decreasing the vital capacity of the lung. Consequently, pulmonary complications such as atelectasis and pleural effusions have been reported5. The higher displacement to failure in the tension band wiring technique suggests the ribs are somewhat less immobilized compared to the other groups. This would be desirable and should reduce the incidence of postoperative pulmonary complications in these patients.
1. Duchenne, G-B. Paris:J-B Balliere, 1862.
2. Bunch et al. JBJS, 75A:372-76, 1993.
3. Thompson, J.D. Manuscript submitted.
4. Ketenjian, A. JBJS, 60A:476-80, 1978.
5. Letournel et al. JBJS, 72A:78-84, 1990.
6. Copeland et al. JBJS, 60B:547-51, 1978.
The authors would like to thank Troy Marlow, M.D. for his help with cadaver bone procurement. We would also like to acknowledge the assistance of Mr. Mark Fielding of the Department of Cell Biology and Anatomy. Without his help, this project would not have been possible.
