Disruption of finger flexor pulleys in rock climbers: prevalence, diagnosis and strategies for rehabilitation - page 4 Strength and efficiency of the flexor pulley system The pulley system of the middle finger is the strongest of the digits, followed by the index, ring and little fingers (Bowers et al 1994; Marco et al 1998). The strengths of the individual pulleys have been extensively studied with varying results (depending on testing protocol), as have the effects of pulley excision. Pulley excision or rupture causes varying degrees of loss of flexion, depending on the extent and position of the pulley rupture. Tropet et al (1990) noted that in a rock climber diagnosed with A2 pulley rupture, active flexion of the PIP joint was impossible. Yet when the affected finger was gripped anteroposteriorly by the examiner, flexion became possible once more. Lin et al (1990) studied the mechanical properties of the pulleys in cadaveric specimens. They found that the maximum breaking strength (Newtons/mm pulley length) were similar for the annular pulleys. However, due to the different lengths of the pulleys, the maximum breaking loads differed significantly. A2 was strongest (407 N) followed by A1 and A4 (209 N). A3, A5 and the cruciate pulleys had much lower breaking loads (<100 N). Load deformation curves were also produced, showing that A2 and A4 tend to be stiffer and less deformable than the other pulleys. An important conclusion from this study was that surgical reconstruction of pulleys should pay attention to pulley position, thickness and length. When pulleys were reconstructed using a ‘belt loop’ technique, near normal breaking strengths could be achieved. Bollen (1990) suggested that the force produced on the pulleys by a 70 Kg man supporting his body mass through one finger using a ‘crimp’ grip would be sufficient to exceed the breaking strengths reported by Lin et al. A similar study (Marco et al 1998) suggested that such an estimate may be an underestimation and the forces produced by supporting body weight may be three times as great as the breaking strengths recorded for pulleys in their cadaveric specimens. However, it is recognised by Marco et al that he age and fragility of the specimens may have adversely affected pulley strength. Marco et al replicated a ‘crimp’ grip and measured breaking strengths of the pulleys with the hand and flexor system essentially intact. The flexor tendons were attached to a loading device and force was applied until failure of all the pulleys and ultimately avulsion of the flexor tendons. With this grip, a distinctive pattern of failure was observed in most cases. A4 ruptured first, followed by A3, A2 and finally the FDS and FDP tendons. A1 did not rupture in any of the 21 fingers tested. Breaking loads were significantly lower that of Lin et al. Lin et al used specifically designed hooks to load the pulleys evenly. In vivo, the forces on the pulleys during ‘crimping’ may be unevenly distributed, creating a ‘cheesewire’ tearing effect. Marco et al suggest that the absence of skin on the volar aspect of the specimens may have reduced the breaking loads observed. These authors also observed that once pulleys A2-A4 had ruptured, the direct transfer of bowstringing force of the FDP tendon of the overlying FDS tendon caused avulsion of FDS. This finding has clinical significance, demonstrating the need for prudent management of pulley injuries in order to prevent the more serious complication of tendon avulsion. Bowers et al suggested that occurrence pulley rupture in vivo is dependent on the degree of flexion of the finger. They suggest that the correct conditions for pulley injury are created when a sudden additional force is applied while the pulleys are already loaded and the finger is flexed to a high degree. In their case study of nine patients with pulley rupture, the A2 pulley ruptured first. Again A1 ruptures were not observed. Several other case studies have suggested that A2 pulley rupture is the most common injury among climbers (Cartier et al 1985; Tropet et al 1990; Moutet et al 1993; Gabl et al 1998). Many of these studies used evidence of clinical bowstringing across the PIP joint as the main diagnostic indicator of A2 injury. However, Marco et al observed that pulley ruptures rarely occurred as isolated events and that clinical bowstringing was only evident after A2-A4 had ruptured, a finding supported by 16 case studies by Martinoli et al (2000). Le Viet et al (1996) observed both isolated A2 and A4 ruptures as well as combined injury to the pulleys in seven patients.
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