IPBSN injury is a common potential complication of operations performed around the knee joint. Because the saphenous nerve and its branches are very close to the incision,and the IPBSN location varies widely cross individuals, even with minimally invasive incision, the saphenous nerve and its branches are still at risk of injury. In recent years, the risk ratio of IPBSN in ACL reconstruction has been reported to be 12–84%[7, 15, 17, 29]. These injuries are mainly related to the harvesting of autologous tendon grafts. In some studies, the authors recommend using an autologous hamstring tendon graft to replace part of the patellar tendon, because hamstring tendon harvesting can reduce the risk of other complications to a greater extent than can patellar tendon harvesting, and the procedure is relatively simple[9, 15, 19]. However, hamstring tendon harvesting can increase the incidence of IPBSN injury.
The saphenous nerve is the longest branch of the femoral nerve and the longest nerve branch relevant to human walking. After this nerve exits the adductor canal, it passes behind the sartorius muscle, and travels along the surface of the gracilis tendon on the postero-medial side of the joint line, and then, it divides into the lower patellar branch and the sartorius branch[5, 11]. This anatomical structure is very close to the hamstring tendon, so this nerve is easily injured when the hamstring tendon is harvested. The sartorial branch of the saphenous nerve extends along the medial part of the tibia toward the distal end of the superficial peroneal nerve on the dorsal side of the second metatarsal bone. The saphenous nerve has only sensory nerve fibers, which innervate the superficial sensation of the medial knee, anterior patella (saphenous nerve, patellar lower limb), medial tibial crest, posterior medial leg and medial foot. It has been reported that injury to the inferior patellar branch leads to the loss of superficial skin sensation, especially in the anterior and medial areas of the patella. Therefore, some patients have difficulty kneeling and walking due to knee-related issues.
The incidence of IPBSN injury has a direct and very important relationship with the type of incision selected for hamstring tendon harvesting. It has been reported that the IPBSN is closely related to the location, type and direction of the incision. In recent years, most studies have reported that oblique incisions are less likely to damage the IPBSN than are vertical and horizontal incisions, because according to autopsy studies, oblique incisions is more parallel and farther away from the IPBSN, so they are located in a safer area[18, 24, 25, 26, 27, 28, 30]. However, horizontal incisions easily lead to injury of the sartorius branch[22, 23].
Portland GHreported that the injury rate related to transverse incision was 43% and that related to vertical incisions was 59%. Papastergiou SG reported an IPBSN injury rate of 14.9% with transverse incision and 37.9% with vertical incisions. A few authors also reported that there are no differences between the two kinds of incisions, and that the injury risk rate is as high as 88%. In recent studies ,the risk of IPBSN injury was lower for oblique incisions than for two other types of incisions[8, 26]. Brandon Michael Henryreported the risk injury with vertical incisions was 64.7%, that with transverse incisions was 50.0%, and that with oblique incisions was 27.6%. Pękala PA et alreported that the risk of injury with vertical incisions was 51.4%, that with oblique incisions was 26%, and that with transverse incisions was 22.4%. All the oblique incisions performed by the authors were COIs, but the reported rates of IPBSN are quite different. According to a study on the anatomy of the IPBSN by Kerver AL et al, a safe area for the IPBSN has been proposed. We found that part of the COI was not located within the safe area, especially when the incision was long. This finding may explain why the rate of IPBSN injury for the COI differs from that for the oblique incision. However, in my research ,the rate of IPBNS injury was 33.3% in the COI group and 9.4% in the MOI group at the final follow-up and the areas of numbness were 26.4 ± 2.4 cm2 and 9.8 ± 3.4 cm2, respectively. Regarding the rate of IPBSN injury or the area of hypesthesia, there was significant difference between groups. Moreover, the type of incision that we proposed is more accurate and easier to perform than that is the COI. Therefore, we think it is a more appropriate method of harvesting hamstring tendons.
According to the literature, blunt injury by a tendon harvesting device is also a cause of saphenous nerve injury. However, it has been reported that there are fewer nerve injuries associated with tendon harvesting than there are nerve injuries associated with incisions[2, 27]. Sanders B et al considered that the tendon harvesting device may only damage the suture branch of the saphenous nerve, while an incision in the hamstring tendon may damage the IPBSN. Almost all the authors of previous studies believe that it is possible to avoid saphenous nerve injury caused by the tendon harvesting device by keeping the knee flexed and thigh rotated during hamstring tendon harvesting, because the saphenous nerve moves backward and is located far away from the incision[5, 21]. In this study, two kinds of oblique incisions were made in this position to harvest hamstring tendons. In our study the results, none of the patients experienced injury to the suture branch, which can cause an abnormal sensation in the medial and distal tibial ridge, as well as sensory abnormalities in the anteromedial skin of the knee joint.
Some authors believe that the length of incision may affect the risk of injury of the IPBSN. Henry BM  conducted an autopsy study and found that in patients who did not experience an IPBSN injury, the distance between various incisions and the IPBSN was very small, with an average distance of 8.2–8.7 mm. Therefore, he believed that the length and direction of the incision were important, and that the length of the incision should be minimized. This also explains why the probability of IPBSN injury was reported to be relatively high (as high as 84%)in the study by Kjaergaard et al, even when an oblique incision was performed. We also found that the COI is not contained completely within the safe area that was proposed by Kerver AL  and Boon , especially when the incision is long. Therefore, the risk of injury to the IPBSN is high. In our study, the average incision lengths for the COI and MOI methods were 3.1 cm ± 0.87 cm and 2.9 cm ± 0.85 cm, respectively, with no significant difference, which reduced the risk of saphenous nerve injury caused by a long incision.
However, a shorter incision makes it difficult to expose the tendon. To expose the tendon, an excessive stretching incision may lead to blunt nerve traction injury. Some authors suggest that surgeons used the blunt separation technique to avoid overstretching and close the wound carefully. This method can also reduce the risk of injury to the IPBSN. In this study, in patients whose level of sensation returned to normal within 3–6 months after surgery, the recovery of sensation was considered to be related to avoid overstretching and close the wound carefully. This study was completed by a single surgeon, and thus, inter-surgeon variability can be excluded. The average length of the incisions was 3.1 cm ± 0.87 cm for the COI and 2.9 cm ± 0.85 cm for the MOI in this paper. We found that all 6 patients with skin sensory loss were obese patients with sensory loss in the lower limbs. The causes of skin sensory loss may be due to the surface markers being placed incorrectly due to difficulty palpating the locations on the body, excessive stretching and a relatively long incision for tendons exposrue.
Although Kerver AL et al and Boon proposed a safe area, the variation in the location of the IPBSN is high. They proposed the concept of a safe area on the basis of a limited number of autopsy studies, which is also a limitation of their work. Therefore, we think that the saphenous nerve injury in patients who undergo the MOI may be related to variations in the location of the saphenous nerve.
In recent years, most of the related studies that have been conducted have reported that regardless of the type incision that was selected, there were no statistically significant differences in postoperative functional scores. Most patients with abnormal sensation around the knee thought that their work and life would not be affected[18, 24, 25, 26, 27, 28, 30]. In this study, there was no significant difference in knee joint function or subjective feeling between the two incision groups.
This study had some limitations. The first limitation is that observer bias may have affected the results. All the data were collected and measured by a staff member. Due to the busy working conditions in our general hospital, no other observers could assist with the study. Second, the sample size is small, limiting the generalizability of the results, and studies with larger sample sizes need to be conducted to confirm the results. Moreover, when the skin incision position was located, we relied on the palpable anatomical structures on the body surface, so there may have been errors in locating the position for each incision, thereby affecting the results.. The third is that due to the small sample size, the results may be biased. Finally, blunt needle acupuncture is not accurate enough to be used to measure the extent of sensory nerve injury. In the future, electrophysiological studies may be used to assess nerve injury more accurately.
All the previous studies have demonstrated that the IPBSN always exists regardless of the type of surgical incision selected and that the incision needs to be performed carefully. Although the MOI performed in this study can significantly reduce the risk of injury, injuries cannot be completely avoided. Mild paresthesia will not affect a patient's life or knee joint function. The area of hypesthesia gradually decreases with time and even recovers completely. Therefore, the risk of postoperative nerve injury should be explained to patients before surgery, but generally, this injury will not affect function.