Radiographic Stress Views in Radial Deviation of the Wrist after Sectioning of Scapholunate Ligament: A Cadaveric Study

Abstract

Dear Sir, An increased scapholunate (SL) angle is one of the diagnostic parameters for evaluating scapholunate interosseus ligament (SLL) lesions. Usually this angle does not appear abnormally raised immediately after the actual injury as this needs the concurrence of several and persistent forces from the moment of the lesion onwards, basically axial loading of scaphoid, to accentuate the abnormal kinematics of the SL joint. A cadaveric study is presented to demonstrate that the torque forces transmitted through the SLL during forced radial deviation are abolished when this ligament is severed, leading to an increased SL angle. This may facilitate the diagnosis of SLL rupture during early stages of the injury by means of a simple radiographic lateral stress view in radial deviation.. One fresh frozen cadaveric specimen was used to carry out the study. Radiographic parameters measured were SL angle in neutral deviation with an intact SL ligament and then several stress lateral views of the wrist in radial deviation in the following three situations: SLL intact, division of the volar component of the SLL and simultaneous division of the proximal and dorsal components of the SLL. Radial deviation of the wrist was carried out manually and forced until an end point was felt. At this stage a radiograph was taken. During the manoeuvre of radial deviation, axial pressure was also applied to maintain the particular congruency at midcarpal and radio carpal levels. Measurement of the SL angles were made from digitalized pictures on computer screen using the tangential method. To carry out this study, initially the SL angle was measured in neutral deviation and it was found to be 53° Then, radiographic stress views of SL angle in radial deviation were also taken and the values did not change with an intact SLL or with a division of its volar component (Fig. 1) These SL angles still measured 53°. However, when proximal and dorsal components of the ligament were severed the SL angle increased to 70° during forced radial deviation. (Figure 2) Fig. 1 SL angle in maximum radial deviation (53°). SL ligament intact Fig. 2 SL angle in maximum radial deviation (70°). Proximal and dorsal SL ligament divided At SL joint level, carpal kinematics can cause two types of strain on the SL ligament: linear strain and torsional strain. When the SLL is damaged, linear strain leads to increased SL gap and torsional strain leads to increased SL angle [1–4]. Torsional forces on the SLL are generated when the scaphoid is brought into flexion and takes the lunate with it. This can be achieved through axial compression of the scaphoid by the trapezium and trapezoid when the wrist is radially deviated . Linear strain on the SLL can be generated also by axial loading of the wrist. This will lead mainly to linear distraction of the fibres of the SLL and to an increased SL gap. Consequently, if the aim is to test the integrity of SLL by the measurement of the SL angle, flexion of the scaphoid through radial deviation is probably the most specific manoeuvre. Based on the findings of this study, when the SLL is intact or only its volar component is divided and the wrist deviated radially, the transmission of torque forces is still preserved through the proximal and dorsal parts of this ligament and the lunate follows the scaphoid into flexion (Fig. 1). However, when all three components of the SLL, volar, proximal and dorsal, are severed, the lunate is unable to follow the scaphoid into flexion because the torque forces have been disrupted, which raises the SL angle significantly because the scaphoid flexes independently (Fig. 2). Further in vivo research is needed to support this cadaveric study. This could be carried out by identifying the possible correlation between radiographic measurement of the scapholunate angle in the stress views presented in this article and arthroscopic or perioperative findings.