A Diagnostic Algorithm to Guide Operative Intervention of Zone 5 Flexor Injuries

Abstract

Background: Given the importance of the neurovascular structures in the volar forearm, accurate diagnosis of zone 5 flexor injuries is critical. Purpose: We sought to test the hypothesis that tendinous injury would be more likely in the distal 50% of the forearm and muscle belly injury would be more likely in the proximal 50% of the forearm. Methods: From December 2015 to December 2016, we conducted a prospective clinical study of patients 18 years and older with zone 5 flexor lacerations. We excluded those with concomitant ipsilateral injuries in flexor zones 1 to 4, multiple lacerations in flexor zone 5, prior neurovascular injuries, crush injuries, patients who underwent operative exploration prior to transfer to our facility, and patients who were unable or unwilling to provide consent. Neurovascular and musculotendinous injuries on physical examination were recorded. All patients underwent operative exploration. Physical examination accuracy and the incidence of musculotendinous and neurovascular injury in the distal 50% of the forearm were compared with the proximal 50% of the forearm. Results: The distal 50% of the forearm (group 1, n = 14) had higher probability of tendon injury (64%), whereas lacerations of the proximal 50% of the forearm (group 2, n = 5) did not result in any tendinous injuries. Rather, all patients in group 2 had muscle belly injuries. There was no difference in the rate of neurovascular injury between groups. Physical examination alone was highly accurate in diagnosing nerve injuries (93%–100%) but less accurate in diagnosing arterial injuries (79%–80%) regardless of the location of injury. Conclusions: Due to the lack of tendinous injuries in proximal zone 5 lacerations, along with the accuracy of physical examination in determining the presence of neurovascular injuries, patients with lacerations in the proximal half of the forearm, without evidence of nerve or arterial injury, can likely be observed in lieu of immediate operative exploration.

Introduction

Flexor zone 5 of the volar forearm is the area close to the transverse carpal ligament. It contains 17 superficial structures: 12 tendons, 3 nerves, and 2 arteries [6]. Due to their proximity to the skin, these structures are susceptible to penetrating injury. Although zone 5 lacerations account for 26% to 48% of all flexor tendon injuries [7,10], little research has focused on this subject [5,8,12].

The close proximity of tendons, nerves, and arteries predisposes patients with volar forearm lacerations to combined injuries involving multiple structures. In addition, the size and mechanism of zone 5 lacerations does not correlate with the number of damaged anatomic structures. As a result, it can be challenging to examine patients with these injuries and observations are often inaccurate, even when conducted by an experienced hand surgeon [4,5]. Due to the unreliable nature of the clinical evaluation, many of these patients undergo operative exploration, only to find irreparable muscle belly injuries proximal to the musculotendinous junction [4].

There is currently no concise algorithm that uses the location of injury to guide surgeons in deciding whether surgical exploration is necessary, or whether clinical examination alone is sufficient to rule out musculocutaneous or neurovascular injury. Accurate identification of the musculotendinous junction is critical, as lacerations proximal to the junction are more likely to involve only the muscle belly, whereas injuries distal to the junction may result in tendon injury. The purpose of this study was to test the hypothesis that tendinous injury would be more likely in the distal 50% of the forearm and muscle belly injury would be more likely in the proximal 50% of the forearm. Secondarily, we sought to compare the diagnostic accuracy of physical examination and incidence of neurovascular injuries in the proximal versus distal 50% of the forearm and develop a diagnostic algorithm to identify which patients may need operative exploration with zone 5 flexor injuries.

Methods

We conducted a prospective clinical study from December 2015 to December 2016. Institutional Review Board approval was obtained prior to initiation of the study, and each patient provided informed consent. All patients 18 years old and older who presented to our urban Level I trauma center with an isolated volar forearm laceration proximal to the transverse carpal ligament were consecutively evaluated for inclusion in the study. We excluded patients with concomitant ipsilateral injuries in flexor zones 1 to 4, multiple lacerations in flexor zone 5, prior neurovascular injuries, crush injuries, patients who underwent operative exploration prior to transfer to our facility, and patients who were unable or unwilling to provide consent.

For each patient who met our inclusion criteria, a preoperative physical examination was performed based on a standardized checklist. The examiners were 1 of 2 orthopedic surgery residents (V.V., A.A.), and they documented demographic information, the mechanism of injury, and whether each of the 12 volar forearm tendons, 3 nerves (ulnar, median, and anterior interosseous nerves), or 2 arteries were injured. Grading criteria were as follows: intact, partial laceration, complete laceration, and unknown. Static 2-point discrimination on each finger was used to determine sensation. Pulse oximetry measurements and the Allen test were used to assess vascular status. In addition to a standardized physical examination, the examiner measured the length (cm) of the forearm and dimensions of the laceration to allow for normalization of the zone of injury relative to forearm length. This value is represented as a percentage (Fig. 1). The examiner was blinded to the physical examination findings of other physicians who evaluated the patient prior to enrollment in the study.

Fig. 1.

The zone of injury was defined as the measured distance of line A over the measured distance of line B expressed as a percentage. This allowed for normalization of the zone of injury relative to forearm length.

Per standard practice at our institution, all patients with flexor zone 5 lacerations and suspected neurovascular or musculotendinous injury underwent operative exploration. At the time of surgery, the operating surgeon, a hand surgery fellow or resident orthopedic surgeon, completed a standardized datasheet, providing detailed documentation of the intraoperative findings and procedures performed. This surgeon was blinded to the physical examination findings of the first examiner.

We elected to stratify forearm regions based on likelihood of tendon injury, based on anecdotal evidence that tendon injuries were suspected to be more common in the distal 50% of the forearm, and muscle belly injuries, which may not need operative exploration, may be more likely in proximal 50% of the forearm. Neurovascular anatomy was not factored into our decision to stratify forearm regions, as these injuries likely require operative exploration regardless of where they occur within the volar forearm.

Based on our hypothesis that zone 5 flexor tendon injuries would be more likely in the distal 50% of the forearm, 2 groups were created. Group 1 consisted of patients with injuries in the distal 50% of the forearm, and group 2 consisted of patients with injuries on the proximal 50% of the forearm. The incidence of neurovascular and musculotendinous injuries was then compared between the 2 groups. Accuracy of physical examination findings between the 2 groups was also compared, using operative findings as the gold standard. Accuracy was treated as a binary variable—either “correct” or “incorrect.” To be labeled as “correct,” the preoperative examination must have correctly identified all neurovascular and musculotendinous injuries subsequently reported during the operative exploration. Finally, we calculated the incidence of tendon injury according to intervals of normalized forearm length and performed a simple linear regression analysis to assess for significance. A 2-tailed Fisher exact test was performed for all categorical comparisons and P < .05 was considered significant.

Results

Nineteen patients with flexor zone 5 lacerations met inclusion criteria and underwent operative exploration. The average age was 38.9 years and 89% were male. The most frequent mechanism of injury was laceration from broken glass, which occurred in 7 (37%) patients. Six (32%) patients incurred self-inflicted wounds. The ulnar artery (42%) and flexor carpi ulnaris (FCU) tendon (37%) were the most commonly injured structures. On average, 2.79 ± 1.54 structures were injured per patient.

Group 1 (distal 50% of the forearm) consisted of 14 patients and group 2 (proximal 50% of the forearm) consisted of 5 patients. Intraoperative results revealed that tendon injury was present in 64% of patients in group 1 and 0% of patients in group 2 (P = .03), whereas muscle belly damage occurred in 43% of patients in group 1 and 100% of patients in group 2 (P = .04). The incidence of tendon injury progressively decreased as distance from the wrist increased (P = .03) (Fig. 2). There were no differences in arterial or nerve injuries between the 2 groups (P > .63). A list of individual musculotendinous and neurovascular injuries is listed in Table 1.

Fig. 2.

The incidence of tendon injury by 20% integral increase in forearm length is represented by the percentage displayed within the forearm. The corresponding number of patients injured in each interval is presented in parenthesis.

Table 1.

The incidence of individual nerve, artery, and tendon damage across patients with isolated flexor zone 5 lacerations.

Structure	Number of patients with injury (%)
	Group 1	Group 2	All patients
Radial artery	1 (7%)	2 (40%)	3 (16%)
Ulnar artery	6 (43%)	2 (40%)	8 (42%)
Median nerve	4 (29%)	1 (20%)	5 (26%)
Anterior interosseous nerve (AIN)	0 (0%)	0 (0%)	0 (0%)
Ulnar nerve	5 (36%)	2 (40%)	7 (37%)
Flexor carpi radialis (FCR)	2 (14%)	0 (0%)	2 (11%)
Palmaris longus	4 (29%)	0 (0%)	4 (21%)
Flexor carpi ulnaris (FCU)	7 (50%)	0 (0%)	7 (37%)
Flexor digitorum superficialis (FDS)	3 (21%)	0 (0%)	3 (16%)
Flexor digitorum profundus (FDP)	3 (21%)	0 (0%)	3 (16%)
Flexor pollicis longus (FPL)	0 (0%)	0 (0%)	0 (0%)

An entirely accurate physical examination was performed in 7 (37%) patients. Specifically, 4 (29%) examinations in group 1, and 2 (40%) examinations in group 2 were correct (P = 1.00). There were no significant differences in the accuracy of the vascular, nerve, and musculotendinous examinations between the 2 groups (P > .53) (Table 2). The vascular examination was correct in 79% of patients in group 1 and 80% of patients in group 2. In all cases of the vascular physical examination being incorrect, an arterial injury was missed on physical examination and diagnosed in the operating room; there were no cases of a suspected arterial injury on examination that was not present in the operating room. The nerve examination was correct in 93% of patients in group 1 and 100% of patients in group 2. The accuracy of the physical examination based on individual arteries, nerves, and tendons revealed that reliable accuracy can be obtained for injury to most important structures. In group 1, injury to the ulnar artery and FCU was more often misdiagnosed on physical examination (Table 3). In group 2, injury to the ulnar artery and flexor digitorum superficialis (FDS) and flexor digitorum profundus (FDP) was more often misdiagnosed on physical examination (Table 3).

Table 2.

The incidence of artery, nerve, tendon, and muscle belly damage based on physical examination and intraoperative findings are presented for group 1 (distal 50% of forearm) and group 2 (proximal 50% of forearm).

	Group 1, %	Group 2, %	P value, %
Suspected preoperative injury
Artery	29	40	1.00
Nerve	50	40	1.00
Tendon	85	40	.08
Muscle	42	100	.045
Intraoperative injury
Artery	50	40	1.00
Nerve	57	40	.63
Tendon	64	0	.03
Muscle	43	100	.04
Correct physical examination
Artery	79	80	1.00
Nerve	93	100	1.00
Tendon	64	60	1.00
Muscle	71	100	.53

If the physical examination corroborated with intraoperative findings, it was deemed to be a correct physical examination.

Table 3.

The percentage of patients with the correct physical examination for each nerve, artery, and tendon across patients with isolated flexor zone 5 lacerations.

Structure	Correct physical examination, %
	Group 1, %	Group 2, %	All patients, %
Radial artery	93	100	95
Ulnar artery	64	80	68
Median nerve	92	100	94
Anterior interosseous nerve (AIN)	93	100	95
Ulnar nerve	100	100	100
Flexor carpi radialis (FCR)	92	100	94
Palmaris longus	91	100	92
Flexor carpi ulnaris (FCU)	77	100	82
Flexor digitorum superficialis (FDS)	86	60	79
Flexor digitorum profundus (FDP)	92	60	82
Flexor pollicis longus (FPL)	92	100	94

Discussion

In an effort to improve the clinical evaluation of zone 5 flexor injuries, we sought to prospectively determine a critical area of the forearm at greatest risk for tendinous injury that would be reproducible in the clinical setting, specifically testing the hypothesis that tendon injuries would be more likely in the distal 50% of the forearm and muscle belly injuries more likely in the proximal 50% of the forearm. The results of this study allow us to accept our hypothesis, as patients with zone 5 flexor lacerations in the distal half of the forearm were at greater risk of flexor tendon injury, whereas patients with lacerations in the proximal half of the forearm are more likely to sustain muscle belly injuries; no patient in the current study had a tendon injury in this group. The incidence of tendon injury progressively decreased as distance from the wrist increased. The incidence and diagnostic accuracy of neurovascular injuries was not different based on location of laceration.

The results of our study must be interpreted within the limitations of its design. First, we only enrolled patients with isolated zone 5 flexor lacerations, thereby creating the potential for selection bias. As a result, our findings may not apply to crush injuries or flexor tendon lacerations across multiple zones. Second, each patient was evaluated by only 1 examiner; therefore, we were unable to assess the inter-rater reliability of the physical examination. Third, our findings are based on a relatively small sample size of 19 patients. Bedside ultrasound was not used in this evaluation; though there can be high sensitivity with identifying flexor tendon injuries with ultrasound, this imaging method is highly user-dependent and may not be available at every institution [13].

Despite these limitations, our study has several strengths. Most important, patients were enrolled prospectively and data were collected using clearly defined physical examination parameters and intraoperative documentation. In addition, although each patient was evaluated by only 1 examiner preoperatively, multiple examiners—all of whom followed the same examination protocol—performed these clinical evaluations during the course of the study. We believe this increases the generalizability of our results. Finally, all enrolled patients underwent operative exploration and therefore had complete intraoperative data, eliminating the possibility of bias based on injury type or severity.

Prior studies have assessed the diagnostic accuracy of physical examinations conducted on patients with zone 5 flexor injuries. In the study by Gibson et al, examiners performed a correct physical examination in only 17% of patients with zone 5 flexor injuries [4]. In the current study, a correct physical examination was performed in 37% of patients, also demonstrating the difficulty of correctly identifying musculotendinous and neurovascular injuries in these patients. The higher number of accurate examinations in our study may be due to our consideration of FDS and FDP as 2 components, whereas Gibson et al treated the 8 tendons of FDS and FDP as individual structures by digit [4]. If 1 tendon in either the FDS or FDP complex is suspected to be injured during the clinical assessment of zone 5 lacerations, operative exploration will likely be recommended; thus, we believe that treating the 2 complexes as composite structures instead of individual tendons is clinically reasonable.

The most common injury missed on physical examination was damage to the ulnar artery. Nerve injuries were least commonly missed. These findings are similar to the study by Gibson et al [4]. The most frequently injured structures were the ulnar artery, ulnar nerve, and FCU, all of which were noted to be disproportionately more frequently injured structures in several prior studies assessing flexor zone 5 [1,9,11,14].

When comparing the 2 study groups, there was no significant difference in physical examination diagnostic accuracy between group 1 (distal 50% of the forearm) and group 2 (proximal 50% of the forearm), nor were there differences in the frequency of nerve or arterial injuries. Patients in group 1 sustained significantly more tendon injuries, whereas all patients in group 2 sustained muscle belly injuries with no damage to the tendons. This result is not unexpected, as most of the forearm tendons originate in the distal 50% of the volar forearm, with the exceptions being pronator teres and palmaris longus [1]. Nevertheless, this is an important clinical finding in terms of zone 5 flexor laceration management. As previously stated, prior studies have recommended the exploration of all zone 5 lacerations due to inaccuracies in the physical examination findings. However, exploration may be unnecessary in patients with isolated muscle belly lacerations, which would not benefit from repair.

Our study demonstrates that the proximal 50% of the forearm resulted in only muscle belly injuries without tendinous involvement. In addition to ruling out tendinous involvement in lacerations to the proximal 50% of the forearm, our study demonstrated high physical examination accuracy for nerve injury, regardless of location of injury. While arterial injuries, particularly ulnar artery injury, were more commonly missed than nerve or tendon injury, this could be diagnosed with advanced imaging, such as computed tomographic angiography, which has high sensitivity and specificity for arterial injuries, with lower cost and patient morbidity than operative exploration [2,3]. Our findings, in conjunction with advanced imaging, can be of significant use in the acute management of zone 5 flexor injuries, particularly when identifying patients who may not require formal operative exploration. A diagnostic algorithm is presented in Fig. 3 based on the results of our study. From a health care resource standpoint, developing an algorithm to identify patients who may not require surgical exploration based on a simple anatomic measurement, physical examination, and a commonly used imaging study may result in less consumption of operating room time and resources, decreased hospital length of stay, and reduced surgical morbidity.

Fig. 3.

Flow chart to guide operative intervention of zone 5 flexor injuries.

In conclusion, musculotendinous injuries due to zone 5 flexor lacerations remain difficult to correctly identify on physical examination. Based on operative findings, patients with lacerations in the distal 50% of the forearm are at greatest risk of tendon injury, whereas lacerations in the proximal 50% of the forearm are likely to affect the muscle belly but spare the tendon. Based on these findings, patients with lacerations in the proximal 50% of the forearm, without evidence of nerve deficits or arterial injury on physical examination or advanced imaging, may not need immediate operative exploration. The diagnostic algorithm proposed needs further validation, but it may lead to improved management of patients with zone 5 flexor lacerations and decreased health care resource consumption.