Proximity of the Femoral Neurovascular Bundle During Hip Resurfacing

The Journal of Arthroplasty Vol. 25 No. 3 2010

Proximity of the Femoral Neurovascular Bundle During Hip Resurfacing Edward T. Davis, MSc, FRCS(T&O),* Price A. Gallie, MBBS, FRACS(Orth),y Steven L. James, MB, ChB, FRCR,* James P. Waddell, MD, FRCS(C),y and Emil H. Schemitsch, MD, FRCS(C)y

Abstract: A standard posterior approach was performed in 5 fresh frozen cadaveric limbs. An anterior incision was then used to measure the distance of the femoral neurovascular structures to the anterior capsule. Magnetic resonance imaging (MRI) of 11 hips was also analyzed to validate our cadaveric method. Cadaveric results demonstrated that before dislocation the femoral artery and vein were closest to the anterior capsule (mean distance, 21 mm), and the femoral nerve was the furthest away (mean distance, 25 mm). These results were not significantly different from the MRI findings. When the hip was dislocated, these distances significantly increased. This study suggests that the neurovascular structures are relatively well protected during an anterior capsulotomy performed during hip resurfacing. The procedure may be safer if the capsulotomy is performed with the hip dislocated and in a flexed position while cutting the anterosuperior aspect and in an extended position while cutting the anteroinferior aspect. Keywords: hip resurfacing, neurovascular anatomy, neurovascular injury, complications of hip resurfacing, hip surface arthroplasty. © 2010 Elsevier Inc. All rights reserved.

Hip resurfacing is becoming an established form of hip arthroplasty in young patients, with increasing literature now available on its' excellent short-term and midterm results [1-3]. One of the technical difficulties when performing the surgery through a posterior approach can be the mobilization of the proximal femur to allow visualization of the acetabulum. To provide adequate exposure, particularly in patients with a tight capsule or in muscular patients, a full release of the anterior capsule is sometimes required to allow the proximal femur to be retracted and to gain circumferential exposure of the acetabulum. Concern has been expressed as to the proximity of the femoral neurovascular bundle during the anterior capsulotomy and the risk of damage during this maneuver. We therefore aimed to identify the proximity of the femoral nerve, artery, and vein during an anterior capsulotomy done during a hip resurfacing procedure using the posterior approach.

From the *The Royal Orthopedic Hospital NHS Foundation Trust, Birmingham, United Kingdom; and ySt Michael's Hospital, University of Toronto, Toronto, Ontario, Canada. Submitted August 17, 2008; accepted February 9, 2009. Benefits or support were received from Smith and Nephew, Memphis, TN. Reprint requests: Edward T. Davis, MBChB, MSc, FRCS(T&O), Consultant Surgeon, The Royal Orthopedic Hospital NHS Foundation Trust, Northfield, B31 2AP Birmingham, United Kingdom. © 2010 Elsevier Inc. All rights reserved. 0883-5403/09/2503-0023$36.00/0 doi:10.1016/j.arth.2009.02.014

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Method We used 5 fresh frozen cadaveric limbs, 3 were male and 2 were female. The mean age was 72.4 years (range, 56-84). Ethical approval was granted for the use of human tissue. None of the cadavers had undergone previous surgery on the hip or lower abdomen. Each limb was placed in the lateral position, and a standard posterior approach was performed. The approach and capsulotomy were all performed by the same surgeon skilled in the procedure (ETD). Before dislocation of the femoral head, the limb was rolled into the supine position and a small longitudinal anterior incision was made to expose the femoral artery, vein, and nerve. Care was taken during this exposure not to mobilize the structures from the surrounding tissues. The limb was then returned to the lateral position. Measurements from the most posterior aspect of the vessels and nerves to the most anterior aspect of the anterior capsule were taken before hip dislocation (Fig. 1). All measurements were taken by one observer (PG) using the depth probe of a Vernier caliper. The tip of the probe was gently placed on the anterior capsule, and the body of the caliper was placed against the vessel or nerve, and the distance was recorded. The femoral head was then dislocated, capsulotomy scissors were inserted though the posterior incision and positioned to cut the superior and then inferior aspects of the anterior capsule. Care was taken to place the scissors on the capsule at the rim of the acetabulum with the scissors manipulated to obtain the “worst case” scenario with relation to the neurovascular

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Fig. 1. Anterior incision to expose femoral neurovascular structures with measurement tool in situ.

structures. The distance from the most anterior aspect of the scissors to the most posterior aspect of each of the neurovascular structures was then recorded. Measurements were made with the hip in approximately 90° of flexion and then in extension (zero degrees of flexion). The hip was placed in maximal internal rotation and neutral abduction/adduction when in extension and maximum adduction and internal rotation when flexed. In an attempt to validate our measurement technique, a separate group of 11 patients were included in the study that underwent routine magnetic resonance imaging (MRI) of the hip for a variety of clinical indications. They included 6 males and 5 females with a mean age of 43.7 years (age range, 18-64 years). A standardized highresolution MRI protocol was used. Magnetic resonance imaging was performed using a 1.5T superconducting unit (Signa LX, GE Medical Systems, Milwaukee, WI). A phased array surface coil (Shoulder Array, Medrad, Indianola, PA) was strapped over the hip and centered over the region of the femoral head. Imaging was performed with the patient supine and the hip resting in extension (zero degrees of flexion). An axial proton density fast spin echo sequence (TR/TE 4000/34) was obtained, and measurements were taken to assess the proximity of the anterior joint capsule to the femoral neurovascular bundle. A workstation was used to review consecutive axial slices, and 3 measurements were obtained at the point where the femoral nerve, artery, and vein were closest in their course to the anterior joint capsule. Measurements were taken from the most posterior aspect of the structure of interest to the anterior margin of the joint capsule (Fig. 2). No significant pathologic condition was identified in any of these examinations, and in particular, the thickness of the anterior hip capsule and zona orbicularis was normal in all cases. The MRI measurements were performed by a specialist musculoskeletal radiologist (SLJ) who had no prior knowledge of the results obtained during the cadaveric dissection.

Fig. 2. Magnetic resonance image scan demonstrating relationship of anterior hip capsule and femoral neurovascular structures.

Statistical Analysis The statistical software package SPSS13 (SPSS Inc, Chicago, Ill) was used. A t test was used to compare the cadaveric and MRI measurements. One-way analysis of variance with Tukey post hoc analysis was performed comparing the differences in distances between the neurovascular structures and the position of the hip. A P value of .05 was used to determine statistical significance.

Results All 5 cadaveric limbs were used. There was no significant difference between the mean distances to the nerve (P = .21), artery (P = .21), or vein (P = .65) between the MR and cadaveric groups (Fig. 3). In the cadaveric group before dislocation, the femoral artery and vein were closed to the anterior capsule (mean distance, 21 mm), and the femoral nerve was the furthest away (mean distance, 25 mm). Superior Capsular Cut After dislocation, there was a significant increase (2531 mm) in mean distance to the femoral nerve when the capsule was cut with the hip in a flexed position (P = .01). There was also a significant increase (P b .0001) in the mean distance to the femoral artery when the hip was dislocated, and the capsule was cut with the hip in flexion (increase in mean distance from 21 to 35 mm) (P b .0001) and in extension(increase in mean distance from 21 to 31 mm) (P = .005). The mean distance to the femoral vein was also significantly increased when the hip was dislocated (P b .0001). When the hip was placed in extension, the mean distance increased from 21 to 31 mm (P = .002) and increased from 21 to 36 mm (P b .0001) when the hip was placed in a flexed position (Fig. 4).

Femoral Neurovascular Bundle  Davis et al

Fig. 3. Box plot showing distances from the femoral neurovascular structures to the anterior hip capsule measured from the cadaveric specimens and from the MRI scans.

Inferior Capsular Cut After dislocation, there was a significant increase (25-31 mm) in mean distance to the femoral nerve when the capsule was cut with the hip in an extended position (P = .005). There was also a significant increase(P b .009) in the mean distance to the femoral artery when the hip was dislocated, and the capsule was cut with the hip in flexion (increase in mean distance from 21 to 27 mm) (P = .019) and in extension (increase in mean distance from 21 to 28 mm) (P = .015). The mean distance to the femoral vein was also significantly increased when the hip was dislocated (P = .014). When the hip was placed in extension, the mean distance increased from 21 to 28 mm (P = .017) and increased from 21 to 27 mm (P = .043), when the hip was placed in a flexed position (Fig. 5).

Discussion Hip resurfacing is becoming an increasingly popular alternate to total hip arthroplasty in the young, active patient [4,5]. Acetabular exposure is more technically challenging than total hip arthroplasty, as the femoral

Fig. 4. Box plot showing the distance from the femoral neurovascular structures to the anterosuperior capsule.

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Fig. 5. Box plot showing the distance from the femoral neurovascular structures to the anteroinferior capsule.

head presents an obstacle to acetabular visualization. To obtain adequate exposure, division of the anterior capsule is often required. This study demonstrates that all 3 femoral neurovascular structures are moved away from the anterior capsule after dislocation of the hip. When cutting the anterosuperior capsule, the distance to all 3 structures significantly increased when the hip was placed in a flexed position. When cutting the anteroinferior capsule in a dislocated position, there was no significant difference in the distance between the neurovascular structures in extension or flexion, although there was a trend for the nerve to be moved further away with the hip in an extended position. Injury to the femoral neurovascular structures after hip resurfacing has been reported. Back et al [6] reported 2 femoral nerve palsies and 1 femoral artery injury from a cohort of 230 patients. They felt that the femoral artery injury was due to a malpositioned acetabular retractor rather than the anterior capsulotomy. Other authors have not described this type of complication; with no reported femoral neurovascular injury after hip resurfacing in a number of other series [2,3,7-9]. Femoral nerve injury has been studied after total hip arthroplasty. In a study of 1000 consecutive hip arthroplasties including 348 primary hip arthroplasties performed through a posterior approach, one isolated femoral nerve palsy and one combined femoral and sciatic nerve injury were reported [10]. The combined injury occurred in a patient who underwent leg lengthening, and the isolated femoral nerve injury occurred in a patient with a femoral fracture. Slater et al [11] measured the pressure adjacent to the femoral nerve during a Watson-Jones approach and confirmed that the highest pressure was seen during the placement of the anterior retractor over the lip of the anterior acetabulum. Weale et al [12] used electrophysiology to study the femoral nerve during 42 total hip arthroplasties (22 posterior approaches, 20 direct lateral approaches).

474 The Journal of Arthroplasty Vol. 25 No. 3 April 2010 They found no clinical femoral nerve injuries. However, they noticed electrophysiologic damage in one patient undergoing total hip arthroplasty through the direct lateral approach. A series of 360 primary hip arthroplasties done though either a direct lateral or transtrochanteric approach reported 7 femoral nerve palsies, all occurring after the direct lateral approach [13]. The same authors undertook an anatomical study suggesting the lesion was due to the anterior retractor being placed over the anterior acetabular rim. We suggest that when hip resurfacing is performed using the posterior approach, the low incidence of femoral nerve and vessel injury, despite the need for the anterior capsulotomy, may be because the anterior retraction of the femoral head moves the nerve and vessels further away from the hip capsule, therefore protecting them from direct injury and/or undue tension. We accept that this study has its limitations due to the small number of specimens used and that the ages of the cadaveric specimens were older than that of a patient usually considered for hip resurfacing. The sample size was limited to 5 specimens due to the availability of cadaveric tissue. We accept that a limitation to this study was that a power calculation was not performed. The cadaveric limbs were sectioned at the level of the lower lumbar vertebra. Therefore, secure fixation of the pelvis was difficult. This resulted in the exact orientation of the hip joint with respect to flexion/extension, internal/ external rotation, and abduction/adduction being difficult to accurately define. The lack of significant hip pathologic condition in the specimens may lead to underestimation of the distance to the neurovascular structures if adhesions between the capsule and muscle are present. The MRI studies were also obtained in patients with a variety of clinical presentations and not in patients before a hip resurfacing procedure. It is not routine practice to undertake MRI of the hip before resurfacing or other hip joint arthroplasty procedures. The mean age in the MRI group was therefore somewhat younger than would be expected for a patient undergoing this procedure, though 6 of the patients were older than 50 years. There was, however, no significant difference between the MRI findings and those obtained at cadaveric dissection. This study demonstrates that the femoral neurovascular structures are relatively well protected during the cutting

of the anterior capsule when performing a hip resurfacing from the posterior approach. We would recommend that the anterosuperior capsule should be cut with the hip dislocated and in a flexed position. The anteroinferior capsule should be cut with the hip dislocated, and it may be safer if the hip is placed in an extended position.

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