Introduction

Leg numbness due to nerve damage is one of the considerable complications after anterior cruciate ligament (ACL) reconstruction using both bone–patellar tendon–bone (BPTB) and medial hamstring tendons. Such patients especially complain of uncomfortable feelings when falling on their knees. Harvesting a BPTB graft includes risks of damaging nerves and causes sensory disturbance.^1,2 Pagnani et al³ pointed out the risk of saphenous nerve damage by harvesting medial hamstring tendons in the region of the pes anserinus.

Many authors have reported the nerve distribution patterns of the infrapatellar regions.^2,4–12 It is well known that both the medial cutaneous nerve of the femoral nerve and the infrapatellar branch of the saphenous nerve are distributed throughout the infrapatellar region and the anterior lower leg region.^9,10,12 The saphenous nerve descends laterally along the femoral artery and enters the adductor canal. It then leaves the artery at the distal end of the canal to proceed vertically along the medial side of the knee and runs between the sartorius and gracilis tendons. In contrast, the medial femoral cutaneous nerve originates from the anterior cutaneous branches of the femoral nerve. The medial femoral cutaneous nerve runs laterally to the femoral artery, and then it crosses anteriorly to the artery at the apex of the femoral triangle to be distributed to the anteromedial thigh and the infrapatellar region. Branches of the medial femoral cutaneous nerve and the infrapatellar branch of the saphenous nerve connect to each other^9,10,12 and form the subsartorial plexus in the infrapatellar region.¹³

Bone–Tendon–Bone Autograft

Anterior knee pain including leg numbness has been reported as a main complication of ACL reconstruction using BPTB grafts. In previous reports, the rate of postoperative anterior knee pain ranged from 4% to more than 40%.^8,14,15 Mishra et al at first described a technique using two horizontal incisions for patellar tendon harvest for the purpose of more cosmetic scarring and reducing pain and flexion limitation.¹⁶ Kartus et al¹⁷ changed to two vertical incisions and reported of an insensitive area compared with the insensitive area that resulted from a traditional vertical incision, which averaged 24 cm. Tsuda et al¹⁸ changed the method of approaching the retinaculum layer, opening it horizontally rather than splitting it to protect nerves using two horizontal incisions, and they reported a 17% rate of postoperative leg numbness. Portland et al¹⁹ compared a horizontal incision and a vertical incision and reported a infrapatellar numbness of 43% resulting from a horizontal incision and 59% resulting from a horizontal incision.

Hamstring Autograft

The donor site morbidity associated with harvesting a hamstring tendon graft is well recognized to be less common than that associated with harvesting a BPTB autograft.²⁰ However, sensory disturbance is frequently observed in regions on the anterior lower leg after ACL reconstruction using medial hamstring tendons.^21,22

Clinical Examination

We clinically examined 103 patients who had arthroscopically assisted ACL reconstructions using medial hamstring tendons to investigate the frequencies and areas of sensory disturbance.²³ As an operative procedure, we made two horizontal incisions for the arthroscopy portal and one longitudinal incision (2.5–3 cm) at the pes anserinus for the tendon harvest and the tibial drill holes. We performed an inside-out technique and used Endobutton (Smith & Nephew Endoscopy, Andover, MA) for femoral fixation. The clinical examination was performed for an average of 13 months (range 6–18 months) after the operation. We detected sensory disturbance on the anterior surface of the lower leg in 60 of 103 (58%) patients. We randomly selected 13 patients with sensory disturbance and neurologically examined in detail the regions of sensory disturbance. The regions of sensory disturbance were of various sizes and shapes (Fig. 75-1). These regions were generally quadrilateral and located lateral to the longitudinal incision for tendon harvest and distal to the horizontal incisions for the arthroscopy portal. In the detailed neurological examination, in 8 of 13 legs (62%) the region was very close to the longitudinal incision (see Fig. 75-1, A to C), and in the other 5 legs (38%) it was relatively far away from the longitudinal incision (see Fig. 75-1, D). The region was lower than the superior end of the longitudinal incisions in 12 of 13 legs (92%); however, in one leg (8%), the region was close to the horizontal incisions (see Fig. 75-1, C). The region was located in the upper half of the lower leg in 10 legs (77%); however, in three legs (23%), the region was wider than others (see Fig. 75-1, B).

Fig. 75-1 Examples of cases of sensory disturbance in 13 legs. A, 4 legs (31%); B, 3 legs (23 %); C, 1 leg (8%); D, 5 legs (38%). P, Skin incision for arthroscopy portal; H, skin incision for tendon harvest.

Anatomical Investigations

In our anatomical study, 51 lower limbs of 26 adult cadavers were used.²⁴ In the mediodistal region of the patella, the nerve branches pierced the fascia cruris in various patterns and ran on the outer surface of the fascia to supply the skin (Fig. 75-2). On the outer surface of the fascia, the nerve branches often were connected to each other. In the regions near the horizontal and longitudinal skin incision lines, the nerve branches ran on the outer surface of the fascia cruris in all legs.

Fig. 75-2 Examples of the distribution of the nerve branches on the fasciae lata and cruris. The nerve branches connect in various manners in the medial infrapatellar region. Some nerve branches pass through the longitudinal skin incision line.H, Skin incision for harvest; P, horizontal skin incision for portal for arthroscopy; Sa, sartorius muscle; Sv, saphenous vein.

After complete removal of the fasciae lata and cruris, the origin of the nerve branches was examined. The anterior surface of the lower leg was innervated by branches of the medial femoral cutaneous nerve proximally and by branches of the saphenous nerve distally. In addition, the patellar region was innervated by branches of the intermediate femoral cutaneous nerve superiorly and by branches of the medial femoral cutaneous nerve medially and inferiorly. Compared with our clinical examinations, the sensory disturbance regions were considered to correspond with the regions supplied by the branches of the medial femoral cutaneous and saphenous nerves.

In 33 of 51 legs (65%; Fig. 75-3, A, B), the infrapatellar branch of the saphenous nerve ran along the inferoposterior border of the sartorius muscle. The infrapatellar branch of the saphenous nerve pierced the distal part of the sartorius muscle in 16 of 51 legs (31%; see Fig. 75-3, C). In two legs (4%; see Fig. 75-3, D), the branch of the saphenous nerve emerged from the anterosuperior border of the sartorius muscle and ran lateralward horizontally. The infrapatellar region and the anterior region of the lower leg were generally supplied by branches of both the medial femoral cutaneous nerve and the saphenous nerve in various patterns. Branches of the medial femoral cutaneous nerve were also distributed to the anterior surface of the leg in eight legs (16%; see Fig. 75-3, A), and branches of the saphenous nerve were also observed to supply the infrapatellar region in two legs (4%; see Fig. 75-3, D). Branches of these nerves and their connections were distributed to the region around the insertion of the sartorius muscle. At least, therefore, the branches of these two nerves showed a complementary distribution and a broad transitional zone. One or more branches of the nerve ran across the line of the longitudinal incision in 88% of the cases.

Fig. 75-3 Four patterns of the distribution of the nerve branches based on findings of 51 legs. A, Branches of the medial femoral cutaneous nerve are distributed to the anterior leg region (8 legs; 16%). B, Branches of the medial femoral cutaneous nerve are distributed to the infrapatellar region, and branches of the saphenous nerve are distributed to the anterior leg region (25 legs; 49%). C, The infrapatellar branch of the saphenous nerve pierces the sartorius muscle and is distributed to the infrapatellar region (16 legs; 31%). D, The infrapatellar branch of the saphenous nerve emerges from the anterior border of the sartorius muscle and is distributed to the infrapatellar region (2 legs; 4%). In, Intermediate femoral cutaneous nerve; Mn, medial femoral cutaneous nerve; Sn, saphenous nerve.

Discussion

In numerous clinical anatomical studies, nerve distribution patterns of the infrapatellar region have been discussed.^{2,4–8,10,25} The nerve branches supplying the skin of the medioinferior region of the patella are recognized as the infrapatellar branch of the saphenous nerve in most studies. However, according to our study and standard anatomy textbooks,^13,16 branches of the saphenous nerve and the medial femoral cutaneous nerve, which originate from the anterior cutaneous branches of the femoral nerve, are distributed to this region. In addition, the branches of the medial femoral cutaneous nerve sometimes extend to the anterior lower leg region. Interestingly, although both nerves are clearly distinguishable in origin, it is very difficult to identify the border between their distribution territories due to their numerous connections because of their complementary distribution.

On the basis of the detailed clinical and anatomical investigations, the sensory disturbance is considered to be closely related to the skin incisions. Nerve injury due to the incisions used for the arthroscopy portal as well as the related anatomical findings have been reported.^2,11 In the present clinical findings, the sensory disturbance region was located in close proximity to the arthroscopy portals in only one case (see Fig. 75-1, C). The branch of the medial femoral cutaneous nerve might have been injured by the incisions for the arthroscopy portal. There have been few reports on nerve injuries related to the skin incision for tendon harvest.¹³ The line of skin incision for tendon harvest at the pes anserinus runs across the nerve branches originating from the medial femoral cutaneous nerve and the saphenous nerve. In some patients, the region of the sensory disturbance is not adjacent to the longitudinal skin incision (see Fig. 75-1, D). These cases might be explained by the various patterns of the connections and the overlapping distribution territories of the saphenous nerve and the medial femoral cutaneous nerve.

The possibility of nerve injury during harvesting of the semitendinosus tendon with a tendon stripper cannot be overlooked. The main trunk of the saphenous nerve runs distally on the medial (outer) surfaces of the tendons of the gracilis and semitendinosus muscles along the medial collateral ligament. Because the sensory disturbance region was located lateral to the longitudinal incision line in all patients, the tendon harvest using a tendon stripper cannot be the main reason for the sensory disturbance. Injury or entrapment of the main trunk of the saphenous nerve has been reported,^26,27 but the sensory disturbance region due to such injury is much wider than that found in our study. If the tendon stripper caused the nerve injury, the main trunk of the saphenous nerve could be damaged as well as the branches of the nerve, as previously pointed out by Pagnani.³ Therefore it is very important to be careful of the main trunk of the saphenous nerve due to its close positional relationship to the tendons to avoid sensory disturbance after ACL reconstruction.

The complicated anatomical variations of the nerve branches in the infrapatellar region and the anterior lower leg region preclude absolute avoidance in any surgical knee incision (Fig. 75-4). Ebrahein and Mekhail⁶ described a safety zone to avoid injury of the infrapatellar branch of the saphenous nerve. However, their zone must be supplied by the branches of the medial femoral cutaneous nerve. Therefore it might be very difficult to find a completely safe zone based on the findings of the present anatomical study.²⁴ Nevertheless, an oblique incision for the tendon harvest would be a good candidate to minimize the area of the sensory disturbance to avoid cutting the courses of the various branches of the nerves, based on our findings of the nerve courses and distribution.

Fig. 75-4 Schematic drawings of the nervous distribution zone in the infrapatellar region and the anterior leg region. The infrapatellar region and the anterior lower leg region are divided into three areas based on the distribution of the nervous branches: X, the region distributed by the medial femoral cutaneous nerve; Y, the transitional zone between the regions X and Z; Z, the region distributed by the saphenous nerve. H, Skin incision for tendon harvest; Mn, branches of the medial femoral cutaneous nerve; P, horizontal skin incision for portal for arthroscopy; Sa, sartorius muscle; Sn, branches of the saphenous nerve.