Posterior Malleolar Fractures
Many authors have reported that the injuries involving a posterior malleolar fragment are associated with a less satisfactory clinical outcome and a higher risk of early degenerative change. The fracture fragment can range in size from a small extra-articular fragment to as much as 40% of the articular surface of the distal tibia.
Several authors have suggested that plain films may either underestimate the size of the fragment and the amount of the joint surface involved or may overrate the size of the fragment, which makes assessment and preoperative planning unreliable.
The use of a CT scan for their preoperative assessment varies widely depending upon the size of the fracture fragment, the quality of plain radiographs available and the surgeon’s preference and experience, however there is an increasing trend of obtaining a detailed account of information prior to fixation of these fractures.
Haraguchi et al. described a method of classification for posterior Malleolar factures according to the stability of the fracture pattern. Their system was based on the orientation of the fracture line as:
- Posterolateral-oblique type (67%)
- Medial-extension type (19%)
- Small-shell type (14%)
Bartonicek et al. used more advanced CT reconstructions to classify posterior malleolus fractures (141 patients). Their system has four main types. They emphasise the importance of recognising fractures that involve the incisura, since accurate reduction will facilitate the reduction and stability of the syndesmosis.
To Fix or Not Fix
Several clinical and biomechanical studies have shown a detrimental effect on the outcome in cases of unfixed or sub-optimally fixed larger posterior malleolar fragments. Macko et al. and Hartford et al. suggested a decrease in the contact area with increase in the posterior malleolar fragment size, the former being responsible for the development of early arthritis.
Fitzpatrick et al. described an anteromedial shift of the contact stresses of the tibiofibular joint with an increasing size of the posterior malleolar fragment, leading to significantly higher stress on the articular cartilage resulting in early arthritis.
Classic teaching suggests that fixation of the posterior malleolus be considered when the fracture size is greater than 25% to 33% of the joint surface. However, a meta-analysis suggests that this was based on low quality evidence and that there is no consensus in the literature regarding the appropriate fracture size for fixation. The arbitrary “one-third rule” was introduced by Nelson and Jensen (1940) who reported on a very small series of patients.
Fixation of the posterior malleolus not only reduces the distal fibula to the correct length through the posteroinferior tibiofibular ligament, but also, ensures the correct ‘working length’ and strength of this ligament and the distal tibiofibular joint, thereby negating any requirement for stabilisation of the syndesmosis.
When the posterior malleolus is addressed surgically, the reduction needs to be accurate. Mal-reduction leads to difficulty in reducing the fibula. It is advisable to fix the posterior malleolus provisionally, and then check that the fibula reduction, before it is fixed definitively. In order to improve the standards of fixation, there is a strong case for pre- and post-operative CT scans to ensure that the goals of the operation have been achieved.
Even after a perfect anatomical reduction of the posterior malleolar fragment, the outcome may not always be entirely satisfactory. Broos et al. reviewed the results of 612 ankle fractures including 172 posterior malleolar fractures and reported that even after a perfect internal fixation, the presence of a posterior fragment larger than one-third of the articular surface led to a worse final outcome than a smaller unfixed fragment.
Different approaches have been described for fixation of the posterior malleolus: the medial approach, posteromedial approach and posterolateral approach, each with its own technical challenges and benefits. Therefore, a CT may help to establish a suitable preoperative plan for achieving satisfactory reduction of specific fracture fragments according to their location.
Most posterior malleolus fractures can be approached using a posterolateral incision, being careful to identify and protect the sural nerve and its branches. A buttress plate or lag screw can be used to fix the fracture, which is biomechanically stronger, and a more logical approach than a screw sited from the anterior tibia, which commonly pushes the fragment away from, rather than lagging it to the tibia.
Fu and colleagues suggested that the most important factor to consider is the location of the fracture fragments because of the deep position and easy rotation of these fragments.
De Vries et al. followed up 45 patients at a mean of 13 years. They were unable to distinguish differences in outcome with respect to the size of the posterior malleolus fragment or to its fixation.
Tejwani et al. studied 456 patients in a retrospective review of 131 patients categorised posterior malleolus fractures as small (< 5% of the articular surface), medium (5% to 25%) and large (> 25%). One year after injury, those with a posterior malleolus fracture scored less well on the Short Musculoskeletal Function Assessment. At a mean follow-up of seven years, they found more arthritis in those with medium or large fragments. The incidence of arthritis was higher where there was articular step-off of > 1 mm. This included over 40% of the patients in whom the posterior malleolus had been fixed.
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Last Updated: March 2018