Accessory Navicular: Excerpt from Book 2: Practical Biomechanics for the Podiatrist

The following is an excerpt from Chapter 6 of Book 2: Practical Biomechanics for the Podiatrist

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Accessory Navicular Syndrome

 

CT scan image of an Accessory Navicular

 

     The accessory navicular is part of the navicular bone, a second ossification center, that never fuses with the main part of the bone. It starts to form around 8-9 years old and is fully formed no later than 16 years old. 10% of your patients will have them and 30% of those will have them on both sides. The prominence caused by this accessory bone has been called the second ankle bone, os tibiale externum, or os navicularis. When they begin to hurt, it is important to actively treat, as a percentage will need surgical removal. The pain can be from the the posterior tibial tendon attachment, from the junction of the accessory and main part (either syndesmosis, synchondrosis, or synostosis), from obvious shoe pressure like from a bunion deformity, or from symptoms arising from the arch collapse. One of the main reasons the medial arch stays strong is the incredible anchoring of the posterior tibial tendon first into the navicular and then spreading out plantarly across the midfoot. If you disrupt this in any way, by having a small amount of the posterior tibial tendon diverted into the accessory bone instead, the foot can strain to hold up the arch and pain ensues. A definite weak spot is created.

     To tie this into Chapter 3 and 4 on gait and biomechanical examinations, when a patient presents with accessory navicular syndrome,  the most important examinations to do in 10 minutes (or 20 minutes) are:

●       Signs of Excessive Pronation and Medial Column Overload

●       Medially Deviated STJ Axis

●       Functional Hallux Limitus

●       Posterior Tibial Strength

●       RCSP

●       AJ Dorsiflexion

 

Common Mechanical Changes for Accessory Navicular Conditions (with the common ones in RED)

1. Cam Walker with or without Crutches in the Immobilization Phase
2.  Inverted Orthotic Devices or another high medial support Orthotic Device (like the Mueller PTTD device)
3. Circumferential Taping with a Hole Cut Out for the Prominence
4. J Strap with Leukotape for Pronation Control
5. Progressive Posterior Tibial Strengthening Program
6. Metatarsal Doming and Single Leg Balancing
7. Strengthening of the 2 Long Flexors
8. 2 Positional Single Leg Heel Raises
9. Off Weighting of the Prominence (including shoe modifications)

 

     Cam Walker with or without Crutches in the Immobilization Phase may be your first treatment. Its mechanical function is in immobilization. If you suspect this problem, you need to err on the conservative side since the problem can lead to surgery if correctly or incorrectly treated. Any patient who first presents with a problem in my practice, that may need a surgical intervention, raises my red flags. You do not know how the injury is going to turn out. Usually looking at the prominence at first sight, before you take x- rays, you are going to know that you are dealing with an accessory navicular (or gorilloid navicular), and that some of these will require surgery. My goal is to get the patient to level 2 pain as quickly as possible on a consistent basis. If that requires a boot and time off work to drive the pain to 0-2, so be it. The goal is to create an environment that will allow them to heal. The crutches may be necessary initially as I experiment with tape, design an insert for the boot, or work on their inflammation. It is important to remember an EvenUp when you use a Cam Walker.

 

Here an EvenUp slips over the side not being immobilized to level the hips some

 

Practical Biomechanics Question #302: 3 patients present with accessory navicular pain with 3 different scenarios. Match the pain with the Phase of Rehabilitation.

1. Pain walking each step
2. Pain only when attempting to run
3. Pain only at 3 miles into running

 

     Inverted Orthotic Devices or another high medial support Orthotic Device (like the Mueller TPD Foot Orthosis) is crucial quickly to stabilize that medial column. Their mechanical function is in reducing pronatory forces on the injured tissue. I am not an advocate of pre-fabricated orthotic devices for children who present with this problem regularly. If a growing child needs an orthotic device, I feel it should be designed for them as exactly as possible. Since most patients who present with significant problems from accessory navicular are juveniles, I discuss with the parents why we have to protect them, even if surgery is needed. I discuss that the presence of this problem will be a weak spot their entire lives and custom support is so crucial. In my book entitled “The Inverted Orthotic Technique” I discuss how this is prescribed.

 

This shows the high medial column support from an Inverted Orthotic Device

Practical Biomechanics Question #303: How much inversion is placed into an Inverted cast to change the foot position one degree?

 

     Circumferential Taping with a Hole Cut Out for the Prominence is typically made from kinesio tape or RockTape perhaps 12-14 inches long and 2 inches wide. Its mechanical function is in light immobilization and support of the injured tissue. Before the tape is placed on the foot, and even without the backing removed, about 1 inch from the start a one inch diamond is cut in its center. The backing is then removed. The one inch diamond cut is placed over the medial prominence as the tape is applied gently over the dorsum of the foot from medial to lateral. When you get to the plantar surface of the foot, the tape is now pulled with “some force” from lateral to medial and up back to its origin and a little further usually slightly more distal (not an exact overlap so it grabs more skin). This type of tape needs to be rubbed in for a minute to activate the glue. You have to play with the tension implied by “some force.” The patient can learn this skill and the tape typically lasts 3-4 days.

 

Leukotape J Strap to create supination moments placed on with the foot slightly inverted and the Coverall protects the skin

 

     J Strap with Leukotape for Pronation Control is the most powerful way of taping for posterior tibial problems, including accessory navicular issues. Its mechanical function is in reducing pronatory moments across the injury and slightly immobilizing. Leukotape is by far the strongest tape I use. It is so strong, it has to have a layer of material called “Coverall” applied to the skin first. No Leukotape should ever touch the skin. Leukotape and Coverall are typically sold together with the Leukotape slightly narrower. Both types of tape of course are applied with the same pattern and it is called a “J Strap”. Occasionally, I tell patients to use 2 layers of Coverall overlapped by 50% to make a wider base to place the Leukotape on with. The tape is started just under the lateral malleolus and brought down and under the heel with no tension, then the foot is slightly inverted to neutral subtalar joint, and the tape is brought up medially over the accessory navicular and up at least 18 inches up the leg. The longer up the leg, the more you are spreading the force to stabilize over a bigger area. Again, like the circumferential taping, this tape should last 3-4 days before being removed. You typically remove it at night, so that you can rest your skin, and re-apply in the morning. It can be used in all of the Phases of Rehabilitation, with some of my recovered patients still using it for long backpacking trips months later.

 

Practical Biomechanics Question #304: Explain why some immobilization of the ankle is needed at times for an accessory navicular problem.

 

Resistance Bands exercise to isolate the posterior tibial tendon with inversion and ankle plantar flexed

 

     Progressive Posterior Tibial Strengthening Program is part of the Re-Strengthening Phase of this injury and vital to its success. Its mechanical function is to strengthen the posterior tibial fibers attaching into the navicular stabilizing the area. Chapter 11 (Book 3) will go through the aspects of strengthening for injuries, so I will just make three points here I want to highlight. First, the goal with all accessory navicular patients, is to progress them to 2 sets 25 repetitions at Level 6 resistance band. This can take months to accomplish so the health care provider has to be clear with the patient that even if they do not hurt, if they remain technically weak, that they can break down again. Second, the posterior tibial nerve which runs right next to the posterior tibial tendon, can make the exercises hurt. I always tell a patient if an exercise hurts they could be hurting themselves. This does not seem to include many patients strengthening their posterior tibial tendon. So, you have to have them do the exercise, and if there is no problem within the first 2 days, have them continue. For that reason, I have them strengthening the posterior tibial tendon every other day initially to check their response. And third, if you rely on anyone else to show your exercises, always check at the first visit. The posterior tibial tendon is strengthened with the ankle plantar flexed (pointed) and the foot inverted (abducted). Over half the time when I check the patient is not doing the exercise correctly.

 

Practical Biomechanics Question #305: What tendon are you strengthening if the foot is inverting against resistance, but the ankle is in neutral to dorsiflexed?

 

     Metatarsal Doming and Single Leg Balancing have both been described multiple times, including the last section on Cuboid Syndrome. Their mechanical function is to stabilize the injured area with muscle strength increases. Typically, with accessory navicular patients, metatarsal doming can be started immediately to keep the foot intrinsics in tone. The Single Leg Balancing is quite jerky and added at the end of the Re-Strengthening Phase or early in the Return to Activity Phase. You typically want the patient at Level 4 or 5 of the resistance bands before starting single leg balancing to ensure that they are strong enough.

 

Single Leg Balancing with Light Touch on Door Frame

 

     Strengthening of the 2 Long Flexors is basically to help the posterior tibial tendon in its ankle plantarflexion and inversion functions, and probably some with arch support. Its mechanical function is to strengthen agonist muscles to the posterior tibial tendon. The posterior tibial tendon, along with the 2 long flexors, run alongside each other under the laciniate ligament under the medial malleolus. They have shared functions, so our strengthening should take some strain off the medial tissues. The classic toe curl exercise, where you build up to 100 curls of the toes as you grab the towel and pull it backwards, is a perfect way to strengthen the two tendons.

 

This is a common gym version where a Bosu Ball is used to balance on single and double legged. The toe flexors can be activated as you lean forward and try to maintain your balance.

 

     2 Positional Single Leg Heel Raises is one of the most powerful exercises you can prescribe and an important monitor of the success of a patient. Their mechanical function is to strengthen the foot and ankle taking stress off the injured area. The ability to do 25 straight knee (gastrocnemius) single heel raises and 12 bent knee (soleus) single heel raises is an indicator of the health of the tissue. However, it is more for the Return to Activity Phase, or later aspects of the Re-Strengthening Phase of Rehabilitation. When the patient presents with accessory navicular syndrome, the testing of whether they can perform a Single heel raise on that side is crucial. It has to be painless. As soon as your heel lifts from the ground, in the next ¼ of an inch of heel rise, the posterior tibial tendon will pull hard on the navicular to assist that heel raise. It is an important overall exercise as the gastrocnemius and soleus supinate the subtalar joint strongly, but it is also an important exercise to sense the strength or frailty of the accessory navicular complex with the posterior tibial tendon.

 

Practical Biomechanics Question #306: What muscles/tendons will help a weak achilles tendon to lift the heel off the ground (any of these structures can be injured because of this)?

 

     Off Weighting of the Prominence (including shoe modifications) is of course really common in ice skaters when they make custom boots and downhill skiers. Its mechanical function is to off weight the sore area. From the age the accessory navicular completely forms, they can become shoe fit nightmares or at least projects. Besides the shoe fitters tasks, on a daily basis the patient may need the use of some ¼ inch adhesive felt to off weight the prominence. The two common ways are 1) a one inch square piece above or proximal to the prominence, and 2) an “upside down smile” making a tent around it, but as close to it except plantarly. This usually is only needed for shoes which seem to bother it. I tell my patients, like my bunion patients, if they remove the shoe at night and the tissue is red at all, the habit of protecting it in those shoes should begin.

 

Here a double layer of ¼ inch adhesive felt is applied proximally in the shoe to off weight the sore point.