Prevalence and Clinical Manifestations of the Anconeus Epitrochlearis and Cubital Tunnel Syndrome

Abstract

Background: The true prevalence of the anconeus epitrochlearis (AE) and the natural history of cubital tunnel syndrome associated with this anomalous muscle are unknown. The purpose of this study was to evaluate the prevalence of AE and to characterize the preoperative and postoperative features of cubital tunnel syndrome caused by compression from an AE. Methods: All elbow magnetic resonance imaging (MRI) scans and all patients undergoing cubital tunnel surgery during a 20-year period were identified and retrospectively reviewed for the presence of an AE. All patients with an AE identified intra-operatively were matched to patients with no AE identified at surgery based on age, sex, concomitant procedures, and year of surgery. Preoperative and postoperative physical exam findings, electrodiagnostic study results, time to improvement, and reoperations were compared between the groups. Results: A total of 199 patients had an elbow MRI, and 27 (13.6%) patients were noted to have an AE present. Average time to improvement after surgical release was 23.0 days for patients with an AE and 33.2 days for patients with no AE. Twenty-seven patients with an AE noted improvement at the first postoperative visit (68%) compared to 15 patients without an AE (33%). No patients with an AE underwent reoperation for recurrent symptoms (0%) compared with four patients (10%) without an AE. Conclusions: The prevalence of AE in our study is 13.6%. These patients experience quicker and more reliable symptom improvement after surgical release than those without the anomalous muscle.

Introduction

The anconeus epitrochlearis (AE) is an anomalous accessory muscle known to compress the ulnar nerve at the elbow leading to classical symptoms of cubital tunnel syndrome. The prevalence of this anomalous muscle is estimated to be 3% to 34% based on cadaveric and imaging studies and is often found bilaterally.^4,10,13 Classically, this muscle was described as an extension of the medial head of the triceps and often presents in conjunction with an enlarged medial triceps muscle.^8,15 In fact, prior reports show AE shortening with elbow extension and relaxation with passive elbow extension suggesting it to be an extension of the medial head of the triceps. In contrast to the triceps, this muscle is innervated by a branch of the ulnar nerve.^7,16,17

When present, the AE muscle occupies more space than the typically present Osborne fascia (Figure 1a).⁹ Patients with cubital tunnel syndrome and an AE accessory muscle improve after release of this muscle (Figure 1b). Prior case reports have hypothesized that ulnar neuropathy associated with an AE muscle may be dynamic and have different characteristics than idiopathic cubital tunnel syndrome including a more rapid progression, younger age of presentation, and shorter duration of symptoms.⁸ Others have suggested that the AE may provide a more forgiving roof to the cubital tunnel syndrome than the more common fascial layer and actually be more protective from developing cubital tunnel syndrome.¹⁹

Figure 1.

(a) Large anconeus epitrochlearis (AE) identified during ulnar nerve (UN) decompression in a patient with cubital tunnel syndrome. (b) After release of the muscle. It was further excised.

The purpose of this study was to identify the prevalence of the AE muscle based on 3-dimensional imaging, and to characterize the preoperative and postoperative features of cubital tunnel syndrome caused by compression from an AE. We hypothesized that patients with cubital tunnel syndrome and an AE accessory muscle would have relatively normal diagnostic tests and a reliably short and complete recovery following surgical treatment.

Material and Methods

A retrospective review was conducted of all magnetic resonance imaging (MRI) studies performed on the elbow in patients older than 18 years between 1996 and 2016 at one institution. The institutional review board granted approval, and all images were reviewed by a musculoskeletal radiologist for the presence of an AE muscle. All patients were included if the MRI was of adequate quality as deemed by the reviewing radiologist. A reviewer blinded to the presence of an AE muscle performed a chart review of all patients included with adequate imaging to identify medial elbow pain, a positive Tinel sign, and electrodiagnostic study results suggestive of cubital tunnel syndrome. All patient information was de-identified and no specific patient consent as obtained.

A second review was conducted to identify all patients who underwent cubital tunnel release between 1996 and 2016 at our institution. Using Current Procedural Terminology (CPT) codes and a billing database, 883 patients were identified who underwent surgery from 1 of 6 different surgeons. Patients below the age of 18 years and who had a release or transposition performed for other reasons than cubital tunnel syndrome were excluded. Using a de-identified copy of main hospital medical record databases created for research purposes, the “Synthetic Derivative” database, 40 charts that referenced the words “Anconeus Epitrochlearis” were extracted (see Supplemental Material). Each case was reviewed for the presence of an AE muscle during surgery. Matched controls with no AE muscle present were identified based on gender, age at the time of surgery, year of surgery, and concomitant surgery (ie, carpal tunnel release, bilateral cubital tunnel release).

Preoperative physical examination features including paresthesia, numbness, intrinsic weakness or atrophy, positive Tinel sign, positive elbow flexion test were identified in the surgeon evaluation of each patient. All patients underwent preoperative electrodiagnostic testing. The presence of fibrillations, sharp waves, and a final interpretation of cubital tunnel syndrome was noted. The same physical exam features and the patient’s or physician’s report of improvement, as documented in the patient’s clinical intake form, surgeon notes, or occupational therapy notes, were recorded at clinic visits 2 weeks, 6 weeks, 3 months, and 6 months postoperatively. The presence and number of reoperations were identified for each patient.

A conditional logistic regression model was performed to examine the association between AE and the electrodiagnostic studies and improvement at the first postoperative visit. To obtain a power of 0.8, a minimum of 27 patients were needed to evaluate improvement at the first postoperative visit and reoperation. Fifteen of 40 patients with an AE and 6 of 40 patients without an AE did not have documented perioperative electrodiagnostic studies and were excluded from analysis of preoperative studies and correlation to the presence of AE. Cox proportional hazards model was used to examine the association between AE and time to improvement. Patients who did not have improvement at their last postoperative visit were from calculation of days to improvement. Proportions were reported as a percentage, parametric continuous variables were reported as a mean and standard deviation, and nonparametric continuous variables were reported as a median and range. For all analyses, P < .05 was considered statistically significant.

Results

Between 1996 and 2016, 207 elbow MRIs were performed and 199 of these were deemed to be of acceptable quality by a musculoskeletal radiologist. Ninety-nine were male and 100 were female. The most common reason for imaging was lateral epicondylitis followed by unspecified joint pain, medial epicondylitis, neoplasm, and ulnar nerve lesion. A total of 27 (13.6%) patients were identified to have an AE muscle. The association of physical examination findings (medial elbow pain, positive Tinel sign) and electrodiagnostic findings for compressive neuropathy to the presence of an AE muscle are listed in Table 1. Notably, there was no significant difference in the prevalence of medial elbow pain (41% vs 38%, P = .77), a positive Tinel sign (3.7% vs 12.2%, P = .19), or negative preoperative electrodiagnostic studies (18.5% vs 18.6%, P = .99) among patients with an AE compared to those without.

Table 1.

Prevalence of AE on MRI.

	Patients with AE	Patients without AE	P value
Sex
Male	18	81
Female	9	91
Total	27 (13.6%)	172 (86.4%)
Presence of medial elbow pain	11 (40.7%)	65 (37.7%)	.77
Positive Tinel sign	1 (3.7%)	21 (12.2%)	.19
Negative electrodiagnostic studies	5 (18.5%)	32 (18.6%)	.99

Note. Prevalence of AE accessory muscle based on all MRI studies performed of adequate quality. The association of AE presence to physical examination findings and diagnostic testing shown for each group. AE = anconeus epitrochlearis; MRI = magnetic resonance imaging.

Over the same 20-year time period, 40 of 883 total patients (4.5%) were identified as having undergone cubital tunnel release and had an AE accessory muscle identified during the operation. Demographics of these patients and the 40 matched control patients with no AE muscle identified during cubital tunnel release are outlined in Table 2. There was no significant difference in age, sex, timing, laterality, or concomitant carpal tunnel release in either group.

Table 2.

Surgical Cohort Demographics.

	Patients with AE	Patients without AE	P value
Average age (years)	48.8	48.6
Sex
Male	29	30	.80
Female	11	10
Year of surgery (range)	2000-2016	2000-2016
Laterality
Right	15	19	.37
Left	20	19	.82
Bilateral	5	2	.24
Simultaneous carpal tunnel release
Male	5	7	.53
Female	6	5	.76
Total	11	12	.80

Note. Demographics of all 40 patients undergoing cubital tunnel release found to have an AE present and the 40 matched control group with no AE. AE = anconeus epitrochlearis.

Preoperative exam findings in patients undergoing cubital tunnel release showed no significant difference between those patients with an AE and those without (Table 3). Approaching significance, more than twice the number of patients without an AE had atrophy compared to those with an AE. Although several patients were excluded and the sample size was small, there was no evidence to show that AE was significantly associated with preoperative electrodiagnostic studies (odds ratio [OR] = 1.50, 95% confidence interval [CI] = [0.25-8.90], P = .66).

Table 3.

Surgical Patient Preoperative Exam and Test Findings.

	Patients with AE	Patients without AE	P value
Tingling	25	23	.65
Numbness	36	39	.17
Weakness	15	15	1.00
Atrophy	4	10	.08
Positive Tinel sign	22	25	.50
Positive elbow flexion test	17	25	.07
Positive electrodiagnostic testing	23	29	.88

Note. Preoperative exam and electrodiagnostic test findings for patients undergoing cubital tunnel release. Forty total patients with an AE and 40 without were identified and reviewed. AE = anconeus epitrochlearis.

After undergoing surgical release, patients with an AE muscle trended toward a quicker recovery with less time to clinically noticeable improvement (23.0 days, SD = 19.0 vs 33.2 days, SD = 20.1; hazard ratio [HR] = 1.54, 95% CI = [0.94-2.52], P = .09). The odds of having improvement at the first postoperative visit in the patients with an AE was 7 times that in the patients without an AE (OR = 7.00, 95% CI = [1.59-30.81], P = .01). Four (10%) patients required a reoperation for recurrent cubital tunnel syndrome when an AE muscle was not present compared to no (0%) patients with an AE present (P = .04) (Table 4). There was no difference between patients who underwent simultaneous carpal tunnel release and those who did not (Table 5). When these patients with concomitant carpal tunnel release were excluded from analysis, patients with an AE were still more likely to have improvement at the first postoperative visit (P = .02).

Table 4.

Outcomes After Surgery.

	Patients with AE	Patients without AE	P value
Average time to improvement (days)	23.0 (SD: 19.0)	33.2 (SD: 20.1)	.09
Number of patients improved by first postoperative visit	27	15	<.01
Number of patients with no improvement	5	10	.15
Reoperations	0	4	.04

Note. Postoperative time to improvement and reoperation rate of all reviewed patients who underwent cubital tunnel release, with and without AE. AE = anconeus epitrochlearis.

Table 5.

Outcomes After Surgery With and Without Concomitant Carpal Tunnel Release.

	Patients with carpal tunnel release	Patients without carpal tunnel release	P value
Average time to improvement (days)	23.6 (SD: 19.0)	29.4 (SD: 21.7)
No. of patients improved by first postoperative visit	14 (60.9%)	31 (52.5%)	.60
No. of patients with no improvement	5 (21.7%)	12 (21.1%)	.95
Reoperations	0	4	.19

Note. Postoperative time to improvement and reoperation rate of patients who underwent cubital tunnel release with (23 patients) and without (57 patients) concomitant carpal tunnel release.

Discussion

Cubital tunnel syndrome is compression of the ulnar nerve about the elbow and is the second most common compressive neuropathy after carpal tunnel syndrome, affecting up to 5.9% of the population.¹ Cubital tunnel syndrome may be caused by entrapment of the ulnar nerve by one of many static or dynamic structures at the elbow, both normal and anomalous. The average medical and wage replacement costs are estimated at $35 000 per patient. The time of lost work after surgical release and anterior transposition is estimated to be an average of 71 days with an estimated cost of disability as $94 per day. This translates into a societal cost of $491.6 million per year. Endoscopic release is reported to significantly decrease the recovery to an estimated 8 days and with potential societal cost savings of $436 million.^6,11,18 Although a popular treatment algorithm in the United States involves open in-situ decompression for cubital tunnel syndrome without nerve subluxation, the societal cost savings and days off work have not been well defined.

The AE has long been thought to be a compressive force on the ulnar nerve and the clinical implications are not well known. Compared to those without an AE, our findings showed that patients experienced a clinically noticeable improvement an average of 10 days sooner if an AE muscle was identified during surgery, although a larger sample size would be needed to attain statistical significance. At the time of the first postoperative visit, more than two-thirds of these patients returned reporting symptom improvement.

Multiple case reports also support a rapid recovery following ulnar nerve decompression in patient with an AE muscle. Nellans et al described a case of a young patient with the presentation of advanced cubital tunnel syndrome as evidenced by atrophy and positive Wartenberg sign.¹⁵ This patient described immediate improvement in sensory symptoms after surgery. In the pediatric population, Boero et al found complete resolution of symptoms and return to full activity after 15 days of casting in a 13-year-old.² Morganstein and Li both described cases of elite athletes and laborers presenting with a dynamic, activity-related cubital tunnel syndrome. All of the patients had return to activity within 3 months and ultimately experienced complete resolution of symptoms.^12-14

Prior reports suggest that cubital tunnel syndrome secondary to an AE accessory muscle is a different entity than idiopathic cubital tunnel syndrome. Erdem Bagatur described their experience with a 28-year-old patient who also experienced immediate relief of symptoms. They hypothesized that the younger age, more rapid progression, and shorter duration of symptoms were related to a subacute process of compression rather than a chronic demyelinating process and were result of a static compression of the nerve regardless of the position of the elbow.⁸ Although this is in contrast to the idea that an AE causes a dynamic compression leading to activity-related symptoms as described by Morganstein, both patient populations had similar positive outcomes.

Although it is known that excision of the AE in a patient with cubital tunnel syndrome leads to symptom improvement, Wilson et al argued that the AE is protective against the development of cubital tunnel syndrome.¹⁹ Their retrospective cohort study identified the AE prevalence as 5.4% in symptomatic patients at their institution while the literature reported prevalence in asymptomatic patients was 15.5%. The authors’ conclusion was that an AE is less rigid than the more commonly present Osborne ligament and contributes to cubital tunnel syndrome only when the muscle is hypertrophied or edematous. Our overall institutional prevalence of an AE was 13.6% compared with a prevalence in operative patients of 4.5%. Our findings would support this conclusion, though when an AE is present in a symptomatic patient, more reliable improvement is expected after surgical treatment. No patients with an AE underwent a revision operation compared with 10% of the control group. In the four patients who underwent revision, one with an in-situ decompression and one with an intramuscular transposition had no improvement with the index procedure, one who had undergone an in-situ decompression had recurrence of symptoms with obvious nerve subluxation, and one had a subcutaneous transposition revised to a submuscular transposition for persistent pain.

Predicting which patients have an AE and can expect sooner recovery preoperatively is difficult based on our findings. Neither physical exam findings nor electrodiagnostic findings correlated to the presence of an AE muscle in those identified on MRI. Cheriyan et al and Byun et al suggested that electrodiagnostic studies were different for patients with cubital tunnel syndrome due to an AE.^3,5 These patients showed a more pronounced velocity drop in ulnar nerve conduction studies or a conduction block of at least 50% (low compound motor action potential amplitude). Other reports highlight the value in preoperative ultrasound to diagnose entrapment and pathologic anatomy.²⁰ Obtaining preoperative MRI or ultrasound for the presence of an AE muscle is not likely to change the decision to intervene surgically; however, this study suggests postoperative counseling of the patient would be different if an AE is encountered. In those situations, a quicker recovery may be seen.

There are limitations to this study. This is a retrospective case-control study and dependent on surgeon reporting. The study was underpowered to show a significant difference in time to improvement (days), though the trend is reported. Given the relative infrequency of an AE in our population, doing a prospective trial would be difficult. Multiple surgeons (>5) at one institution were included in this study and each had variable detail about the presence, size, and appearance of the AE. Similarly, improvement was judged based on the documentation of the treating surgeon regarding patient-reported symptoms and physical exam, which was variable. No standardized patient-reported outcomes were used to determine the degree or complete resolution of symptoms. The potential recall and interviewer bias may decrease the validity of this study. Although our study suggests improved patient outcomes if they have an AE present, future studies should address the ideal treatment of cubital tunnel syndrome caused by AE and if surgical intervention for these patients should involve excision of the AE, excision and transposition, or in-situ decompression.