Posterior Ankle Impingement
Posterior ankle impingement syndrome (PAIS) is a term that describes pain in the back of the ankle when plantar flexing maximally, and sometimes when dorsiflexing. There are several reasons this can happen and can be a result of a bony process, unfused ossicles (small bones), or soft tissue (muscle/tendon/ligament/capsule). It is common amongst football players, soccer players, ballet dancers, and downhill runners. There are other nervous system dysfunctions such as tarsal tunnel syndrome that have to be considered and ruled out. To understand the mechanism that causes pain, we first have to understand the relevant anatomy of the ankle.
The ankle is a hinge joint and it is comprised of three bones, the tibia (shin), the fibula, and talus. The socket of the joint is shaped more like a bracket and is also called the ‘ankle mortise’ because of its shape.
The movements of the ankle mortise primarily dorsiflexes (toes to the nose), or plantarflexes (stepping on a gas pedal). As the ankle dorsiflexes, the talus has to roll posteriorly, and as it plantarflexes, the talus rolls anteriorly. That is to say that the talus moves in opposite directions like a socket joint.
Under normal circumstances, when talus glides anteriorly, it is not problematic. However, if there is tissue present that is not supposed to be there, it gets pinched in the joint space between the talus and the tibia.
Bony anatomy can cause posterior ankle impingement. This can be loose bodies floating between the talus and the tibia, anatomical variance in the shape of the talus, or from fused bones. Between the ages of 7-13 years old, an ossification center forms at the talus and usually fuses within one year. Sometimes it forms a large process on the lateral aspect of the talus and is termed a ‘Stieda process’. If it fails to fuse, it becomes a free floating bone called “os trigonum’ (Robinson & White, 2002). 25% of the population have os trigonum and pain is caused by the talus and tibia compressing the loose bone like a nutcracker. Both of these scenarios can cause pain in the posterior ankle when plantar flexing.
Soft tissue structures involved in posterior ankle impingement can be muscle, the posterior capsule of the joint, or posterior ligaments. The muscle that flexes the big toe (flexor hallucis longus) runs between the lateral and medial processes of the talus and with os trigonum or a Stieda process, the space becomes narrowed and causes irritation of the sheath of the tendon (Robinson & White, 2002). It is important to note that this is a separate condition from Achilles tendon pathology.
Most people respond well to physical therapy for posterior ankle impingement and one small study found success rates for conservative rehabilitation to be approximately 60% (Yasui et al., 2016). A treatment plan will include restoring range of motion, strengthening the ankle, and addressing capsular restriction if indicated. It will also include education in limiting excessive plantar flexion. In high level athletes that don’t respond to physical therapy, surgery may be indicated. Surgical options may include removal of the lateral Stieda process, os trigonum, or division of the tendon sheath if it has thickened (Wredmark et al., 1991). For athletes that want to return to sport promptly, surgery has the advantage of being minimally invasive with low rates of post-surgical complications, and short recovery times. However, the downfall is that the procedure is technically difficult and if the athlete has anterior ankle pathology (which many do), the procedure becomes even more challenging (Wredmark et al., 1991).
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