Sling Stirrups Leading to Femoral Nerve Injury

Subject

Sling style and Allen-type stirrups can both be used for positioning in gynecologic surgery. Surgeons must be aware of the potential for nerve injury related to patient positioning.

Case

A 58-year-old G3P3 presented with pelvic organ prolapse. The patient was noted to have second-degree uterine prolapse, pop-q stage 2 cystocele, rectocele, and stress incontinence on presenting examination. She desired definitive management and underwent vaginal hysterectomy, bilateral salpingectomy, transobturator tape sling, anterior/posterior repair, uterosacral ligament plication, and perineorrhaphy. Sling stirrups were used for lithotomy surgical positioning.

The total documented operative time was 4 hours and 6 minutes. In the recovery room patient had bilateral hip pain R > L, numbness on bilateral lateral thighs, and full movement of bilateral lower extremities. Leg weakness was noted on postoperative day 1 and neurology was consulted. On physical examination, patient had diminished sensation to pinprick and light touch on bilateral lower extremities with left side diminished S1, L3, and L4; and right side involving L3 and L4 distribution. Patient also noted to have 4/5 strength in bilateral lower extremities with hip flexion and knee extension, and 4/5 strength with adduction/abduction. Their final diagnosis was L3–L4 neuroplexopathy likely secondary to femoral nerve entrapment bilaterally. They anticipated that this would resolve during the postoperative period and the patient was discharged with a walker for ambulation. At the patient's postoperative visit 2 weeks after the surgery, she had complete resolution of the symptoms and was walking independently.

Review

Surgeons must be aware of the importance of safe patient positioning, and care must be taken to optimize visualization during surgery, but also minimize risk of adverse events related to prolonged lithotomy position. Patients under anesthesia have muscle relaxation that prevents them from using protective reflexes, and this decreased muscle tone raises their risk of joint luxation, plexus injuries, and pressure ulcers. Around 30% of all patients complain of back pain postoperatively with up to 37% of patients complaining of back pain after undergoing surgery in the lithotomy position. ¹

Some risk factors reported for neuropathy postoperatively are body mass index (BMI) ≤20 kg/m², prolonged duration of surgery in lithotomy position, excessive pressure on patient's thigh during surgery, diabetes mellitus, and history of smoking. ² Nerve injuries can be classified into three categories depending on extent of damage to the nerve. These categories include neurapraxia, axonotmesis, and neurotmesis. Neurapraxia occurs with mild compression or traction on the nerve leading to muscle weakness. Axonotmesis is direct damage to the axons with focal demyelination with maintenance of the nerve's connective tissue. The last and most critical injury is known as neurotmesis, which is full transection of the axons and connective tissue. ³

The most common nerves injured in the lithotomy position are the common peroneal and femoral nerve. The femoral nerve can be injured by abdominal retractors in open surgery, or by hyperflexion and compression under the inguinal ligament while the patient is in stirrups. This compression of the femoral nerve can be worsened further by the surgical assistant leaning against the inner aspect of the thigh during surgery. The peroneal nerve can be injured either by lateral compression at the fibular head, or nonphysiologic traction of the sciatic and peroneal nerve with hips hyperflexed and knees extended in the stirrup position. ¹

Patient positioning is a modifiable risk factor in surgery that can decrease the incidence of neuropathy postoperatively. The difference between sling stirrups and Allen-type stirrups has been recently investigated. A randomized control trial, published in August 2020 in the Obstetrics and Gynecology Green Journal, compared these two stirrup types used in a variety of vaginal surgery, including any combination of vaginal hysterectomy, vaginal vault suspension, vaginectomy, midurethral slings, and others. Postoperative evaluation of physical function was conducted using a validated survey of physical function. The conclusion of this randomized trial of 138 women, was that the use of Allen-type stirrups resulted in better physical function outcomes 6 weeks postoperatively compared with sling stirrups. ⁴

This study has findings consistent with what is to be expected, but there are also some limitations to the conclusions that were made. The sling stirrups have more variation in the positions that can be obtained with positioning. There is a risk of excessive relaxation leading to increased hyperflexion, movement in Trendelenburg, and movement with assistants leaning on patient during cases where sling stirrups are used. The Allen-type stirrups are fixed in position, so would have less likelihood of moving during the case unintentionally. There is some argument that the sling stirrups allow better visualization and vaginal access; however, this needs to be weighed against the risk of nerve injury.

Limitations to this study have been discussed by other physicians in the field of obstetrics and gynecology. ⁵ Although the study was appropriately designed, there are flaws that raise question as to how to interpret the results. The first issue is whether the femur angles, which were statistically significant in their differences between stirrup types, relate to physical function. It could be extrapolated that the wider the angle of the femurs the increased abduction and external rotation; however, the study does not explain why this has a role in physical function outcomes at 6 weeks. This would need to be further investigated to pinpoint the cause of worsened physical function, as the specific issue of neurologic dysfunction was present equally in each of the study groups. The other criticism is that femur angles are not inherent to the type of stirrup used, and both stirrups can achieve the same range of angles depending on positioning. The final issue is that the angles were measured before surgery without repeat measurements at the conclusion of the procedures. Patients move during cases and the legs are adjusted as well; therefore, it would be useful to know the final angles to assess more accurately.

The study made conclusions that angles were wider in sling stirrups, which led to worsening physical outcomes at 6 weeks. To evaluate if femur angles versus stirrups were the issue at hand, the same stirrup should have been used and various angles measured to see if this caused a difference in outcomes. Having varied the angles and stirrups leads to two separate variables that may be implicated in outcomes, which is not helpful in attributing a true cause–effect relationship. Regardless of stirrup type, physicians should ensure that there is not excessive pressure on the lateral fibular head, hip flexion >90° should be avoided or time in this position limited, and the abduction angle with flexion of this hip should not exceed 45°. ¹

Trainee Perspective

Patient positioning is a critical component of surgery that should be an essential part of residency training and education. Iatrogenic nerve injury is an adverse effect that can be prevented with adequate knowledge, appropriate use of positioning aids, and a thorough understanding of the anatomy and pathophysiology of nerve injury in surgery.

The likely cause of this femoral neuroplexopathy was entrapment of the femoral nerve at the inguinal ligament for a prolonged duration indicating a Grade I nerve injury. The design of sling stirrups poses an increased risk to this type of nerve compression. This is likely attributed to the potential for excessive flexion of the thigh with abduction and external rotation of the hip that can occur with improper positioning in these patients. ²

Neuropathy events have also been reported with Allen-type stirrups as well. The most common nerve injury associated with the lithotomy position is peroneal nerve injury. This is thought to be secondary to limited soft tissue cushioning at the fibular neck and can be addressed with adequate padding. ⁵ The Allen-type stirrups especially pose a risk for this type of injury.

As I reviewed this case, and evaluated my experiences, it is clear that there needs to be more information before a recommendation between these types of stirrups can be made. During my medical school training it was common to see sling stirrups being used, but when I came to residency I have seen almost exclusively Allen-type stirrups. This is important because different training programs have varied exposure to positioning techniques and equipment. Although I think the previously cited randomized trial was examining an important topic in gynecologic surgery, I think the conclusions they gathered do not help to target incidence rates of serious neurologic injuries such as the presented case. Another critique of the study is that patient positioning regardless of the type of stirrups used is dependent on the person that is positioning the patient.

Both sling and Allen-type stirrups can be used inappropriately and pose potential risks for injury that is not inherent to the stirrup type used. When I leave residency and am practicing on my own, I will take personal responsibility for the positioning of my patients ensuring that they are safely positioned to decrease the risk of nerve injury during procedures. I do not feel that either type of stirrup is necessarily superior to the other but since I have trained and am more comfortable with Allen-type stirrups, I would use the technique I was most comfortable with. The key point to take from this is that physicians should be knowledgeable and well trained to position their patients and use the equipment available to decrease risk of iatrogenic neuropathy during gynecologic surgery.

Faculty's Perspective

Patient positioning is fundamental to gynecologic surgeries. Residency provides trainees with exposure to a variety of techniques and preferences for nearly every detail of patient care, and patient positioning is no different. Although the incidence of neuropathies is low, an injury obviously has a significant impact of patient function and satisfaction.

A significant strength of this study is its randomized, single-masked design. However, this study was limited to 6 weeks after surgery. There was also insufficient power to draw conclusions about neuropathies being more common with sling stirrups. It is also important to note that measurements were not obtained at the conclusion of a case, and positioning can obviously change during a procedure. Regardless of the type of stirrup, inappropriate positioning is a risk for injury. Trainees should learn how to appropriately position patients regardless of stirrup type. Further investigation is important to compare stirrup types with an adequately powered study as well as assess long-term outcomes.