Surgical anatomy for a new minimally invasive approach to harvest the flexor digitorum longus tendon: A cadaver study
Article Outline
Abstract
Aim
The purpose of this cadaver study was to test feasibility and safety of a new technique for harvesting the FDL tendon through a plantar incision placed directly overlying the FDL division and to define the relevant surgical anatomy.
Methods
In eight cadaver feet the FDL tendon was exposed in the midfoot through a plantar incision. In four of these feet this was done after localization of the FDL division site using a probe inserted through a wound in the hindfoot within the sheath of FDL tendon. The exact location was measured in relation to the geometry of the foot and was found to be midway between the back of the heel and the base of the second toe and about 4
cm medial to the lateral border of the foot. Using these determinants FDL was exposed on the contralateral matching four feet without use of the probe. The FDL tendon was divided and pulled proximally and the length of the tendon was determined.
Results
The FDL division lies midway between the back of the heel and the base of the second toe and about 3.7cm medial to the lateral border of the foot. The medial and the lateral plantar neurovascular bundles are, respectively, about 0.43cm and 0.86cm away from the FDL division. The average length of the retrieved tendon was about 9cm.
Conclusions
This cadaver study suggests that the FDL tendon can be safely harvested through a plantar incision. The adjacent neurovascular structures remained undamaged. Plantar surface anatomy guides placement of the plantar incision so that the incision can overlie directly over the FDL division.
Keywords: Flat foot, Flexor digitorum longus, Tendon, Planovalgus foot deformity, Posterior tibial tendon
1. Introduction
The flexor digitorum longus (FDL) tendon is used for reconstruction of dysfunctional posterior tibial and Achilles tendons [1], [2], [3], [4], [5], [6]. The surgical technique to harvest this tendon was described by Mann and Thompson [1]. They advocated a medial-based approach and a dissection which proceeded along the FDL tendon in the deeper layers from medial border of the foot towards laterally in the midfoot. This medial approach, however, requires extensive, deep and difficult dissection in the midfoot in vicinity of blood vessels and nerves. There is risk of injury to plantar neurovascular structures [7], [8], [9].
A plantar approach being more directly overlying the FDL tendon may limit the dissection and the risk of injury to the neurovascular structures. Anatomy in the midfoot and relationship of the FDL and the flexor hallucis longus (FHL) tendons has been described in the literature [9], [10], [11], [12]. But surface anatomy to help locate a plantar incision over the point where the FDL tendon divides has not been described before.
The purpose of this cadaver study was to evaluate as follows, a new plantar approach to harvest the FDL tendon. (1) To define the optimum location of a plantar skin incision which can overlie and give a direct approach to the distal most end of the FDL tendon before it divides into its slips for the toes. (2) To assess feasibility of cutting the FDL tendon through this approach and harvesting the cut end through a wound in the hindfoot. (3) To assess any damage to the adjacent neurovascular structures.
2. Materials and methods
Eight fresh frozen cadaver legs (four right and left pairs) were brought to room temperature by thawing them overnight. None of the legs had any previous surgery or had an obvious deformity. In four right-sided specimens the following technique was used to harvest the FDL tendon. The feet were dissected to expose the tendon sheath of the FDL behind the medial malleolus. A malleable metallic probe with a smooth bulb at its tip was then introduced within the tendon sheath and passed gently distally into the midfoot (Fig. 1). A vertical incision was made in plantar aspect of the midfoot over the prominence made by the tip of the probe. The skin was incised to expose the vertically oriented fibers of the aponeurosis which were separated to expose muscle fibers of the flexor digitorum brevis muscle. These muscle fibers were separated and retracted to expose the FDL tendon (Fig. 2). The identity of the tendon was verified by applying a pulling tension on the tendon through the proximal wound in the hindfoot and assessing transmission of the tension distally to the tendon identified in the midfoot and at the same time observing flexion in lesser toes. The FDL tendon was then cut sharply at the level where it divided in the midfoot and the cut end pulled proximally through the exposure in the hindfoot (Fig. 3, Fig. 4). The length of the plantar skin incision was measured.
Fig. 1.
A malleable probe inserted in the FDL tendon sheath through the wound in the hindfoot.
Fig. 2.
The hemostat is underneath the FDL tendon which is exposed through the plantar incision. The tip of the probe inserted is visible in the plantar aspect of the foot.
Fig. 3.
The FDL tendon being pulled through the hindfoot after it has been cut in the midfoot.
Fig. 4.
The cut FDL tendon harvested through the hindfoot.
An ink mark was made on the FDL tendon at a level horizontal with the tip of the medial malleolus and from this mark the length of the FDL tendon, up to its cut end was measured. Then further dissection was carried out in layers to raise rectangular flaps of soft tissues surrounding the plantar incision in the midfoot and path of dissection traced, to expose the division of the FDL tendon and adjacent structures including the neurovascular bundles (Figs. 5–7). Further measurements were obtained as follows. The flat surface of a metal plate was held against the back of the heel perpendicular to the plantar aspect of the foot to define the back of the heel. A ruler was held parallel to the plantar surface of the foot and the distance between the back of the heel to the proximal flexor crease at the base of the second toe was measured. Then the distance between the back of the heel to the point of the division of the FDL tendon was measured. Next the distance between the FDL division and the adjacent medial and lateral neurovascular bundle was recorded. The flat surface of a metal plate was held against the lateral border of the foot. The perpendicular distance between this flat surface and the FDL division was noted. All linear measurements were taken to the closest 0.5mm. The adjacent medial and lateral plantar neurovascular structures were dissected to look for injury.
Figs. 5–7
Successive layers of soft tissue reflected to expose the FDL tendon and adjacent neurovascular structures.
In the four left-sided specimens the measurements obtained on the plantar surface of the corresponding right-sided specimens were used to guide the location of the plantar incision and approach to the FDL tendon division. The malleable probe was not used to guide the location of the plantar skin incision. The rest of the dissection and measurements were carried out as in the right-sided specimens.
3. Results
In all of the eight cadaver specimens it was possible to reach the FDL tendon division through the plantar incision. In all specimens the cut end of the FDL could be withdrawn from the wound in hindfoot. However, in two specimens that were a right and left pair, a slip between FDL and flexor hallucis longus had to be cut before this was achieved. This slip could be visualized in the plantar exposure when the FDL tendon was pulled towards the toe and the big toe plantar flexed. The slip was cut through the same exposure with out need for extension of the plantar wound.
The average length of plantar incision in eight specimens was 3.33cm, in right-sided specimens it was 3.32cm and in the left-sided specimens it was 3.35cm. The average length of FDL tendon graft in all eight specimens measured from a level horizontal with tip of medial malleolus to the cut end of the tendon was 9.04cm.
The measurements of the different parameters studied in eight specimens are given in Table 1.
Table 1. FDLd—point of flexor digitorum longus tendon division
| Distance between points/parameter measured | Average±S.D. (cm) |
|---|---|
| Heel to base of second toe | 20.1625±1.48 |
| Heel to FDLd | 10.375±0.67 |
| Base of second toe to FDLd | 9.7875±1.15 |
| FDLd to lateral border of foot | 3.72875±0.61 |
| FDLd to lateral plantar nerve | 0.865±0.44 |
| FDLd to medial plantar nerve | 0.4375±0.36 |
| FDLd graft length | 9.04±1.10 |
4. Discussion
In the cadaver specimens studied it was feasible to harvest the FDL using the minimally invasive approach and exposure in the plantar aspect of the foot, without causing visible injury to adjacent neurovascular structures. Surface anatomy can help localize the point where FDL divides.
Sullivan and Aronow studied the relationship of medial plantar nerve to the FDL tendon by dissecting nine fresh frozen cadaver feet [12]. They found that the medial plantar nerve crossed the FDL tendon at a variable distance from the medial aspect of the talonavicular joint. This measured between 4mm and 31mm from the medial border of the talonavicular joint with an average of 6mm. They also found that the medial plantar nerve courses inferior to the FDL tendon proximal to the Henry's knot therefore allows for safe exposure both dorsally and medially to the FDL tendon at this level. They believe that harvesting the FDL tendon distal to the Henry's knot is associated with risk of injury to the nerve [12]. Harvesting this tendon distal to the Henry's knot may, however, be necessary to obtain a graft of sufficient length [8], [12]. The minimally invasive plantar approach is advantageous in this regard, as it provides access to the very distal end of the FDL tendon thus giving maximum possible length of the graft.
The medial-based approach is associated with risk of injury to the medial plantar nerve [7], [8], [9]. Transection of medial plantar nerve injury can lead to loss of sensation, loss of function and cock up deformity of the toe [7]. In addition to the risk of nerve injury, there is morbidity associated with the dissection necessary to harvest the FDL tendon through a medial approach. Mann and Thompson described this approach where the incision is along the medial border of the foot [1]. The FDL sheath is identified at the medial malleolus and followed distally. The fascia of the abductor hallucis muscle is incised and the muscle is retracted plantarward to expose the interval between the flexor hallucis brevis and the first metatarsal. The origin of the flexor hallucis brevis muscle is released to further enhance the exposure on the plantar aspect of the foot. The FDL tendon sheath is opened and the tendon kept on tension, the dissection of the sheath is carried distally and deeper into the plantar aspect of the foot. As this is achieved there are quite a few vessels that need cauterization. The FDL tendon is sharply divided at the distal most extent possible [1].
The longer the length of the graft sought, the wider and deeper is the approach and dissection necessary to provide adequate visualization in the depths of the wound. Adequate visualization is also necessary if tenodesis is attempted between the FDL stump and the FHL tendon to prevent inadvertent damage or a suture into adjacent neurovascular bundles.
This medial or ‘open’ approach therefore requires extensive dissection in the midfoot in vicinity of blood vessels and nerves. This is because a structure in a deep layer, near the middle of the midfoot, is being approached from medial border of the foot. On the other hand in the plantar approach, the incision and dissection directly overlies the end of the FDL tendon and therefore is ‘minimally invasive’ and is likely to be associated with lesser morbidity and lesser risk of nerve injury.
The plantar approach described here would be even more advantageous in situations where a medial incision in the midfoot is not required. For example, when the FDL tendon is used in repair of the lateral ankle ligaments or the peroneal tendon or the Achilles tendon.
Attachments between FDL and FHL tendons have been described [11]. In two specimens in this study it was necessary and possible to identify and cut the attachment between these two tendons, before the cut FDL tendon end could be harvested.
The limitations of this study are those associated with a cadaver study. The measurements based on soft tissue landmarks are subject to an error. However, a plantar incision based on such soft landmarks on surface of the foot is reasonably accurate in giving access to the deeper structures of interest such as distal end the FDL tendon before it divides. Also the incision, itself being in soft tissue, can be manipulated to bring it over the area of dissection.
Again, although the measurements reported here between the FDL tendon and adjacent neurovascular structures are subject to inter- and intra-observer errors and errors also due to manipulation during cadaver dissection, these measurements can serve as a guide. And this guide can be helpful during dissection to locate these structures and to retract them out of harms way.
A longitudinally oriented plantar scar in the midfoot may be a cause for concern when this minimally invasive method is used in patients. However, the resultant scar is likely to be at the junction of the middle and medial third of the foot and in the relatively less weight bearing part of the foot. The incision used for the excision of a plantar fibroma is similar in its vertical orientation and location and the length much longer than would be necessary for the plantar approach to the FDL tendon. Therefore a resultant scar from this minimally invasive plantar approach is likely to be tolerated equally or better than after other procedures requiring a plantar incision.
Clinical studies are necessary to evaluate this ‘minimally invasive’ plantar approach and compare it with the currently used medial ‘open’ approach in terms of ease of technique, length of incision required, length of graft obtained and outcomes including associated morbidity and nerve injury.
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PII: S1268-7731(07)00073-2
doi:10.1016/j.fas.2007.08.003
© 2007 European Foot and Ankle Society. Published by Elsevier Inc. All rights reserved.
