Key Pearls For Performing Bunion Surgery
When performing bunion surgery, one must keep in mind various biomechanical considerations. These authors discuss factors such as the intermetatarsal angle and metatarsal declination as well as the importance of pronation and first ray hypermobility.
Bunions occur in 0.89 percent of the population and the incidence increases with age.1 About 6.5 percent of people 18 to 44 years old have bunions and this increases to 16.2 percent in the 45- to 64-year age group. For those over 65, the bunions can occur in over 22 percent of the population. Females have the condition at a ratio of approximately 3:1 over men, which most likely is due to footwear.1
The etiology of bunions is multi-factorial. Root and colleagues describe four types of conditions (biomechanical, arthritic, neuromuscular and traumatic) that may lead to hallux valgus deformity.2 Factors such as shoe gear and genetics play indirect roles.
Bunions were originally thought to be a swelling of the soft tissue or an enlargement of the first metatarsal head, or possibly both. The early bunion surgeries aimed at removing the underlying bursa alone or in combination with an exostectomy of the first metatarsal head. This type of surgery was beneath many of the surgeons of the times so the understanding of the pathology and development of new surgical techniques were slow in occurring. In 1881, Reverdin presented the first real surgery to attack hallux abducto valgus.3 The Reverdin procedure is considered to be the forerunner of all the newer procedures we know today.
Geometry is critical in podiatric surgery. Many of the procedures that we use currently for different pathologies are determined by biomechanical angles. In particular, bunion surgeries are determined by the angle of the first metatarsal relative to the second metatarsal and the hallux itself. After determining what procedure to perform, there are many geometric concerns that we need to be aware of when performing bunion surgery.
What You Should Know About The Intermetatarsal Angle
The intermetatarsal angle is an important consideration when determining what type of bunion surgery to perform. For example, when performing a closing base wedge osteotomy of a transverse orientation, it is important to figure out the size of the wedge one should remove to bring the first ray into better alignment. The goal of this osteotomy is to make the first and second metatarsal approximately parallel.
One can determine the amount of wedge to resect before surgery by assessing the intermetatarsal angle. After measuring the intermetatarsal angle, utilize the law of similar triangles to figure out the size of the wedge that you should remove. The law of similar triangles states that if two angles of one triangle are congruent to two angles of another triangle, then the triangles are similar.4 Therefore, using this particular geometric proof, the wedge removed should be “similar” to the predetermined intermetatarsal angle.