INTRODUCTION
Lateral and posterolateral approaches to the elbow are often used to expose the proximal radius during surgical management of elbow fractures or arthritis. The posterior interosseous nerve (PIN) crosses the lateral aspect of the radius from proximal and anterior to distal and posterior directions and is at risk of iatrogenic nerve injury during elbow surgery [
1-
3]. In the literature, forearm pronation is the reason for distal PIN movement from the radial head (RH) and out of the surgical window [
1,
4]. Therefore, the recommendation is to maintain the forearm in a pronated position during elbow surgery when lateral or posterolateral approaches are used. Previous studies were conducted on cadavers [
1,
4-
7], with dissection of soft tissue surrounding the PIN; however,
in vivo migration of the PIN across elbow and forearm positions is unclear. We hypothesized that the PIN moves away from the RH during forearm pronation. The purpose of this study was to clarify the
in vivo dynamic migration of the PIN across elbow and forearm positions using ultrasound imaging.
METHODS
The Institutional Review Board of Teikyo University reviewed and approved the study protocol (No. 20-133) and waived the requirement for written informed consent. This study included 43 upper extremities with a full range of motion in 22 healthy volunteers (16 females; mean age, 29 years). The exclusion criteria were subjective or objective muscle weakness, sensory disturbance, or history of extremity trauma, which was the case for one extremity. The subjects were asked to sit in front of an examination table with their shoulder slightly flexed in an internal rotation with the lateral epicondyle on top. The examiner was an orthopedic surgeon with 15 years of experience in musculoskeletal ultrasound. We measured the shortest distance from the RH to the point where the PIN crossed the lateral aspect of the radial axis in six positions of the elbow and forearm (90° forearm supination, neutral, or 90° forearm pronation, each at 135° of elbow flexion and 0° of elbow extension). All included extremities met the range of motion criteria of 0° to 135° and achieved 90° of supination/pronation. The measurements were obtained using ultrasound imaging with an 18 MHz linear-array transducer (HI VISION Preirus, Hitachi Aloka Medical) (
Figs. 1 and
2). Continuous variables are shown as means±standard errors. The distances at each elbow and forearm position were compared using paired t-tests. Statistical significance was set at P-value <0.05. Basic demographic data are presented in
Table 1.
RESULTS
The RH-to-nerve distance was greater during elbow extension than flexion in each forearm rotation position (
Fig. 3). At elbow flexion, the head-to-nerve distance was significantly greater during forearm pronation (23.4±0.84 mm) than during supination (21.6±0.76 mm, P=0.010). In contrast, the head-to-nerve distance was not significantly different between forearm pronation (25.1±0.68 mm) and forearm supination (25.1±0.79 mm) during elbow extension (P=0.980). The maximum migration distance was 3.5 mm and occurred when transitioning from elbow extension and forearm pronation (25.1±0.68 mm) to elbow flexion and forearm supination (21.6±0.76 mm). All relevant data and the differences between the dominant and non-dominant arms are presented in
Table 2.
DISCUSSION
The main finding of our study was that the RH-to-nerve distance associated with the elbow extension position was greater than the distance associated with elbow flexion, independent of forearm rotation. In general, the elbow and forearm positions in which the RH-to-nerve distance was longer appeared safer because of reduced risk of injury resulting from the nerve emerging in the surgical window. However, when examining the RH-to-nerve distance changes by position, the difference between the extended pronation position and flexion supination position was the largest at 3.5 mm but may be inconsequential to surgical risk.
In the literature, forearm pronation resulted in distal movement of the PIN from the RH, excluding it from the surgical field. Hackl et al. [
1] examined the location of the PIN in neutral rotation, supination, and pronation using three-dimensional x-ray scans in six upper extremities of fresh-frozen cadavers. In the sagittal view, the PIN crossed the proximal radius 61.8 mm and 41.1 mm below the RH in pronation and supination, respectively. Diliberti et al. [
4] showed in a study of 32 fresh cadavers that the PIN crossed the proximal radius 52.0 mm and 34.4 mm from the radiocapitellar articulation in pronation and supination, respectively. However, flexion and extension of the elbow did not affect these distances. Calfee et al. [
5] reported that the PIN crossed the radius at a mean of 5.6 cm distal to the radiocapitellar joint in pronation, and supination decreased this distance to 3.2 cm in 20 fresh-frozen cadaveric upper extremities. Overall, the evidence indicates that pronation increased the proximal safe zone to approximately 2 cm in the lateral approach. In contrast, our
in vivo study indicated that PIN migration during the same movement was 3.5 mm, which is significantly less than that reported in cadaveric studies.
The PIN branches separate from the radial nerve at the arcade of Frohse and continue along the supinator muscle of the forearm, under the brachioradialis muscle. Therefore, full extension may exert tension on the PIN, while elbow flexion may relieve this tension. Full flexion, which contracts the anterior muscles, may push the PIN further toward the radiocapitellar joint. However, in our vivo study, PIN migration was minimal in each forearm position. This finding may be due to the supinator and brachioradialis muscles ensuring a constant position of the PIN in vivo. In contrast, in cadaveric studies of PIN migration, sufficient dissection of the soft tissue and muscles is performed to clearly visualize the position of the PIN. Soft tissue dissection may allow the PIN to move more freely than it does in vivo.
This study has some limitations. First, the PIN originates from the posterior humerus and travels to the anterolateral forearm. The angle of the PIN along the proximal radial shaft results in a significant deviation of the RH-to-nerve distance depending on ultrasound probe placement. We did not consider intra- or inter-examiner errors or individual differences in sex, age, or body size, which may have affected the presented estimates. Second, most elbow surgeries are performed in the midrange of flexion at approximately 90°. However, in our study, measurements were performed only in full extension and near-full flexion, in which range the PIN distance is unclear. Third, the potential effects of muscle tension cannot be ignored because the measurements were obtained in an awake patient. In addition, the effects of injury could not be assessed due to the in vivo nature of this study.
CONCLUSIONS
We found that the PIN moved farther from the RH during elbow extension than during elbow flexion and farther during forearm pronation than during forearm supination. However, distal migration of the PIN was minimal, indicating that a difference of just 3.5 mm may not guarantee safety during surgery. Thus, care should be taken to ensure that the PIN is not within the surgical window even when the forearm is in the pronated position.