Anatomy and histology of the scapholunate ligament

The scapholunate ligament links the scaphoid to the lunate. It runs transversally at its posterior aspect and obliquely at its anterior aspect, allowing significant relative motion between the two bones. From the neutral position to the full extension position, the lunate rotates by 28 degrees and the scaphoid by 30 degrees; from the neutral position to the full flexion position, the lunate rotates by 30 degrees, whereas the scaphoid rotation is 60 degrees because of the motion of the scaphoid around the capitate. The ligament's dorsal part is shorter and more resistant than the anterior part, allowing a pseudodissociation during flexion. Kauer described an additional movement of the scapholunate pair attributable to differences in the shapes of the scaphoid and lunate proximal poles. The scaphoid curve is more important and the scaphoid needs to glide on the lunate to maintain radioscaphoid congruity. As a result, there is sagittal ligament torsion. This can be a partial explanation for failure of scapholunate arthrodesis. This description of the scapholunate ligament is of interest to understand the relative importance of the three parts of this ligament. It can, in particular, explain the failure of ligamentous reconstruction that considers the scapholigament as a homogeneous structure. In addition, the three parts do not have the same tensile strength. The posterior part is the most resistant to tear forces and needs more than a 300 N tensile stress to fail. The anterior part fails with 150 N stress and the intermediary portion can withstand only a 25 N to 50 N stress. In comparison, the triquetrolunate ligament (which is also divided in three parts--anterior intermediate, and posterior) has failure coefficients opposite those of the scapholunate ligament: The anterior part is more resistant (300 N) than the posterior (150 N); the intermediate part has the same tensile strength as the scapholunate intermediate part. These biomechanical studies demonstrate the importance of the scapholunate ligamentous posterior part in controlling flexion and extension motion and the anterior part for rotational control. Both parts of the ligament are necessary for an harmonious functioning of the scapholunate pair.