Introduction
Submandibular gland excision is a commonly performed procedure by the otolaryngologist. The most frequent indications are non-neoplastic conditions such as chronic sialadenitis and sialolithiasis. Traditionally, surgery involves a direct transcervical approach. This places the marginal mandibular branch of the facial nerve at risk of injury, either directly or more frequently through traction. Reported rates of paresis are as high as 36 per cent for temporary paresisReference Smith, Peters and Markus1 and 12 per cent for permanent paresis.Reference De, Kumar Singh and Johnson2
Careful placement of the skin incision, along with a detailed knowledge of the region anatomy, is crucial in avoiding injury to the nerve. Surgical texts differ in their description of incision siting (Table I).Reference Gleeson, Browning, Burton, Clarke, Hibbert and Jones3–Reference Thumfart, Platzer, Gunkel, Maurer and Brenner6 In practice, an often-quoted technique is two fingerbreadths below the mandible. Having casually observed an obvious difference in the size and the breadth of surgeons’ fingers, we postulated that this might not be a consistent or accurate method of measurement, and its use may put the marginal mandibular nerve at risk of injury. We conducted a study to assess how frequently the technique is utilised, and to evaluate and quantify differences in finger size.
Suggested landmarks for submandibular surgery incision
Materials and methods
Prior to an explanation of the project's objective and methods, we questioned 28 otolaryngology consultant and middle grade surgeons (in the Northern Ireland region), who had been trained in salivary gland surgery, as to their method of marking the skin incision location in submandibular gland excision. Subsequently, we used Vernier calipers to measure across the breadth of the middle and index fingers of consultants and all surgical trainees available within the departments. Measurements were recorded at the level of the mid skin crease of the proximal interphalangeal joint of the middle finger. Measurements of both dominant and non-dominant hands were recorded.
Results
Of 28 surgeons questioned, 23 (82 per cent) use the 2 fingerbreadths below the mandible method to mark the incision site for submandibular gland surgery. Two surgeons (7 per cent) specified the tips of the fingers. Other methods described were: a tape measurement of 2.5 cm below the mandible (n = 2; 7 per cent); and thumb width measurements below the mandible (n = 1; 4 per cent), at the level of the hyoid (n = 1; 4 per cent) and in a skin crease (n = 1; 4 per cent).
We obtained the bilateral middle and index finger measurements of 42 surgeons (12 female and 30 male). Breadth ranged from 3.1 to 4.7 cm (mean, 4.0 cm). Overall, there was an average of 1 mm difference between dominant and non-dominant hands (4.1 cm vs 4.0 cm respectively). Male surgeons’ fingers were significantly broader (mean, 4.2 cm) than those of female surgeons (mean, 3.6 cm) (Mann–Whitney U test; U = 55, p < 0.001) (Figure 1).
Box plot of fingerbreadth measurements of male and female surgeons.
Discussion
The submandibular gland lies between the horizontal ramus of the mandible and the underlying mylohyoid and hyoglossus muscles. It is deep to the platysma muscle and is enveloped in a fine capsule derived from the investing layer of deep cervical fascia. The marginal mandibular branch of the facial nerve lies closely adherent to the outer aspect of the investing fascia. It innervates the depressor anguli oris muscle. Cadaveric anatomical studies have examined the relationship and distance of the nerve from the inferior margin of the mandible. Wide variability in location exists. In up to 66 per cent of cases, the nerve lies below the mandible.Reference Rödel and Lang7 The lowest position identified is up to 3 cm from the inferior border of the mandible.Reference Wang, Lin, Kuo and Shih8
Like patients, surgeons come in all shapes and sizes, including the breadth of their fingers. This is particularly noticeable with an increasing number of women entering surgical training (27 per cent of surgical specialty training year 1 acceptances in 2007, compared with 8 per cent of consultant surgeons).Reference Elston9 This could in theory have implications for the use of the fingerbreadth technique for incision placement in submandibular gland surgery.
We are the first authors to have objectively measured variation in surgeons' anatomy. The results revealed an inconsistency and large variation in the size of surgeons’ fingerbreadth measurements, with the smallest two fingers being measured at 3.1 cm and the largest at 4.7 cm. However, based on cadaveric studies, we have in fact proven that the technique remains adequate for incision placement in submandibular gland surgery. Even for the smaller handed surgeon, the incision will be located below the lowest recorded site of the marginal mandibular branch of the facial nerve. Although the fingerbreadth measurement is a safe technique for placing the incision, only sound anatomical knowledge of the submandibular region will minimise the risk to the patient's marginal mandibular nerve and reduce the chance of a poor cosmetic outcome.
We have highlighted the significant variability between individuals (maximum difference of 1.6 cm in our small series). We believe that surgeons should be aware of what two fingerbreadths actually means in relation to their own hands. This may be particularly relevant in the training environment, where a large inconsistency could arise between a trainer's expectation of incision placement and that actually performed by a trainee.
