What is Sinus Tarsi Syndrome?
The sinus tarsi is a bony canal inside the foot. It runs roughly crossways (like left to right) just in front of the outside ankle bone. If you feel that area, you will find a deeper spot where you can push your finger further inwards than in the bony areas that surround it.
The canal is formed by two ‘half-pipe’ like grooves carved into the bones that meet there – the Talus on the top and the Calcaneus (heel bone) on the bottom. Inside the canal is a ligament, blood vessels, nerves, fat and some gristle-like tissue that binds the bones together.
The joint that is made up by those two bones is called the subtalar joint. This joint sits ~3 cm below the ankle joint and, where the ankle joint can only move in one plane (up and down like a nodding head), the subtalar joint is where your foot can move in every plane. This allows it to make circular rotations and to cope with uneven terrain underfoot.
In true sinus tarsi syndrome, the tissues in the canal will show laboratory signs of inflammation, cellular damage and fibrosis – an overgrowth of tissue similar to scar tissue.
The underlying cause of the damage might be:
• a tear of the ligament that binds subtalar joint together
• damage to the cartilage of the joint (an osteochondral defect)
• injury to the subtalar joint, especially where the joint ends up less tightly bound together allowing too much motion to occur
• degenerative disease of the subtalar joint
• a birth condition that binds the two bones together too much
• long term inflammation of the soft tissue within the sinus
What are the signs and symptoms of Sinus Tarsi Syndrome ?
Pain will be felt in the location of the sinus tarsi, near the outside ankle bone as described above. Walking on uneven ground is usually painful and often there is a history of ankle injury. To test for the condition, pain is usually worsened by standing on the feet and tipping the soles inwards – like a sprained ankle position. The pain will be temporarily relieved by injecting local anaesthetic into the canal. This is a diagnostic test – not a treatment – as the effect only lasts a few hours.
X rays of the joint are generally normal unless the deterioration is caused by something that is visible on films, such as degenerative arthritic disease. MRI is the investigation of choice and will show the soft tissue conditions well if there is significant inflammatory and fibrotic changes. It also allows a good view of the cartilage and bone components of the joint which is difficult to see on plain x ray as the joint’s curves make it difficult to get a good shot ‘through’ it.
How do you treat Sinus Tarsi Syndrome ?
Treatment would usually start with providing mechanical support to the foot to control ‘bad’ movements in the subtalar joint and address poor function of the tendons around the ankle (see a podiatrist). If the condition has been present only a short time and might be transitory, oral pain killers and anti-inflammatory agents may help. If this does not have the desired effect, steroid injected into the joint should be considered as the next step. There are surgical options for the recalcitrant cases which could mean an open incision or surgery by arthroscopy. A 1999 study (by Frey) indicated largely good results via the keyhole method.
If not Sinus Tarsi Syndrome, what else might it be ?
It would be fair to say that true Sinus Tarsi syndrome is quite uncommon. There is, however, a very common complaint that creates pain in the area of the Sinus Tarsi that is not the condition described above. The subtalar joint, which contains the sinus tarsi canal, is a complex joint with three articular facets. This means that three pairs of cartilage coated bony areas rub together during each part of the joint’s movement. The bottom bone is shaped a bit like a horse’s saddle and the top bone is equally contoured in the reverse way. It is difficult to imagine the joint at work, but some idea of the concept will help to understand this other condition.
Imagine that you went to the gym and picked up a heavy hand weight. If you did bicep curls the right way – without straightening the elbow all the way out – the session should pass without incident. If instead of this, you flung your arm straight out at speed until your elbow joint was forced to stop the movement because you have reached the point where bone hit on bone, you can see that you would hurt yourself. This is a good analogy for what happens in the subtalar joint. If your foot posture and function is bad (almost always over-pronation tipping in the heel bone), instead of working in the middle of the range of motion of the subtalar joint, you work at the end. Each time you do this, it ‘pinches’ the joint on the outside edge and causes pain in the region of the sinus tarsi. Over time, the joint can develop serious wear and tear which means osteoarthritis in medical terminology. This condition doesn’t have a single agreed on common name but is probably best termed Lateral Subtalar Joint Impingement. The good news with this condition is that treatment is simple, non surgical and has very good rates of success. As with all degenerative conditions, the earlier the treatment is commenced, the less damage has time to build up so see your podiatrist without delay.