An Anatomical Study of Nerves at Risk During Minimally Invasive Hallux Valgus Surgery

Hallux valgus is a very common pathology affecting the first toe. The first metatarsal is deviated medially, while the first toe is deviated laterally. Minimal invasive surgical correction implicate ostomies which are performed through a small incisions of less than five millimeters are located on the medial and dorsal side of the foot.

This allows introduction of this instruments that you can see here, but don't allow visualization of the narrow vascular structures addressed during incision placement as it happens during open surgery. For this reason, we have performed anatomical study to assess nerve position related to this incision. Thorough anatomical knowledge is essential in any surgical field, but especially during minimally invasive procedures.

In the case of hallux valgus surgery, the dorso medial nerve of the first toe, branch of the superficial peroneal nerve, and the dorso lateral nerve of the first toe, branch of the deep peroneal nerve, are at risk. The purpose of this study was to assess the position of the nerves, related to surgical incisions, and to re-straight this with a new method easy to remember for the surgeon. In order to perform with accuracy in an anatomical study, it is necessary to be trained in anatomical dissection.

We use only fresh frozen specimens, with are thawed in water at room temperature for eight hours, before starting the dissection process. We dissect using a technique of plane per plane anatomical dissection, respecting all the structures encountered. In order to perform this anatomical study, the material needed includes latex gloves, a number four handle with 24 blades, tooth forceps which are used to handle the skin, non-toothed forceps for the internal tissues, a periotome like the ones used in dentistry, iris scissors, needles to fixate the nerves, a standard goniometer to perform measures, and some gauze pads to clean during the dissection process.

We carry out the study of this area, because here is where incisions for hallux valgus minimally invasive surgical procedures are located. The medial incision at the first metatarsal phalangeal joint to perform the bone resection and ostomy, puts at risk the dorsal medial nerve of the first toe. The dorsal incision at the distal part of the first inner metatarsal space to perform a tanatomy of the adductor hallucis muscle, puts at risk the dorsolateral nerve of the first toe.

The first step of the study is to dissect and locate the pertinent neurological structures, and fixate them so they won't move when we carry out the frontal section. Dissection starts with an incision cutting only the skin, without affecting the subcutaneous tissue. Skin is removed exposing the adipose subcutaneous tissue, which is very important to be left intact to avoid altering nerve position, as cutaneous nerves are located inside this tissue.

If location of the nerve is doubtful, iris scissors are very useful, as they allow to locate it without over dissecting the area. This is the dorsal medial nerve of first toe, which is distributed over this area. As veins are more superficial than nerves, they can be removed without risk of affecting the results of our study.

After locating the dorsal medial nerve we will start the dissection to locate the dorsal lateral nerve of the first toe, which we know is to be found at the first intermetatarsal space. It is important to note that the technique of anatomical dissection is usually carried out with the blade not facing the specimen but facing out. That allows us to remove part of the tissue without affecting the underlying structures, in this case a vein, which is ultimately removed.

This is the base of a true plane per plane anatomical dissection. Gauze pads are used to clean part of the dissection area, especially when blood makes difficult the visualization of the structures. Again, iris scissors are useful when dissection with a cutting blade may put at risk the structure we want to locate.

The periotome is useful to remove the adipose tissue that is typically around the nerves, and allows its correct identification. Finally, the dorsal lateral nerve of the first toe is identified. And dissection continues until the point where the nerve arrives to the metatarsal fallangio joint.

When identification of the dorsal lateral nerve of the first toe is established, fixation of the nerves with needles is started. Traction of the toe helps to locate the exact position of the joint line. Then, the nerve will be fixated with a needle located intra articularly, to avoid its movement.

The dorsal medial nerve is fixated with a needle, and the same process is followed with the dorsal lateral nerve. Now, the nerves are fixated by the adipose subcutaneous tissue, and by the needle, which assures that it will not suffer any movement during the freezing and cutting processes. The specimen is protected into plastic, to avoid cold burning.

And then, it's frozen at minus 17 degrees, for a minimum of six hours. Before performing the sawing process, the frozen specimen is sectioned in a diamond saw, with water refrigeration to avoid burning the structures due to friction. The frontal section is made just posterior to the metatarsal phalangeal joint as this is the zone where the surgical incisions are made.

The saw allows performing sections as thin as needed, and in this case to avoid movement of the structures, we perform one centimeter sections. When the section is ready, measurements are obtained with a standard goniometer, and then extrapolated, to a Clock Method, in which the sphere is the contour of the first metatarsal diaphysis. The Clock Method has been previously used in other joints, like shoulders or knees, but it is the first time used in foot surgery.

The dorsal medial nerve was found on an average of 26.2 degrees medial to the medial border of the EHL. Using The Clock Method, the dorsal medial nerve was found in all cases between 12 o'clock, and two o'clock, corresponding to zero and 60 degrees, respectably in a right foot, and between 10 o'clock, and 12 o'clock in a left foot, corresponding to 300, and 360 degrees, dorsal medial aspect of the first toe. The dorsal lateral nerve was 32.3 degrees lateral to the medial border of the EHL in average.

The dorsal lateral nerve was found in all cases between 10 o'clock and 12 o'clock in a right foot, and 12 o'clock and two o'clock in a left foot, dorsal lateral aspect of the first toe. The dorsal medial and the dorsal lateral nerves of the first toe are consistently found between 10 and 2 o'clock. These are the safety area where incisions for Hallux valgus minimal invasive procedures can be performed without risk.