This study was approved by our Institutional Review Board. Data from all patients treated during the study period are available for review and analysis. All participants provided informed consent prior to data collection. This study used a convenience sample of individuals attending clinics at our university hospital and consecutive patients that met the inclusion criteria were invited to participate in this study.
Patients
Among the patients with CuTS who visited our institution for treatment between April 2010 and August 2017, we excluded those with secondary associated pathologies. Thirty-nine patients with idiopathic CuTS (18 male, 21 female; mean age, 47.8 years (SD 11.8)) were assigned to group A, and 40 patients without CuTS symptom (20 male, 20 female; mean age, 42.5 years (SD 8.8)) were assigned to group B (control).
Group A included patients with ulnar nerve injury (McGowan grade 2, n = 31; McGowan grade 3, n = 8) with muscle weakness, pain, numbness, or paresthesia in the distribution of the ulnar nerve and a positive result in Tinel’s nerve percussion test and electromyography. An experienced neurologist performed all examinations and revealed a substantially delayed motor nerve conduction velocity in the ulnar nerve segment crossing the elbow. Twenty-eight McGowan grade 2 patients and 8 McGowan grade 3 patients underwent cubital tunnel release, and all patients had symptom improvement. Three McGowan grade 2 patients who were lost to follow-up were excluded.
Group B (control) included patients without symptoms of CuTS who visited the hospital because of elbow pain that required CT imaging (in group A, CT was performed for simple elbow joint pain); both groups were matched for age and sex. None of the patients had systemic diseases that might have contributed to the occurrence of neuropathy or more proximal compression lesions.
Imaging techniques
Patient positioning
Measurement with CT may vary because of tilt vibration resulting from changes in the posture of patients or varied positions of the elbow inside the machine [19]. Thus, an arm support system was prepared for the capturing of images in all patients, ensuring consistent positioning inside the CT machine; adjustments were made for consistency with the posture during x-ray imaging of the cubital tunnel. The arm support system consisted of a tilted wooden bar (20°) and a handgrip on a flat wooden plate. Subjects were positioned nearly prone on the device and were asked to grab the arm support system with the forearm supinated, shoulder externally rotated at 20°, and elbow full flexed (Fig. 2).
CT imaging data acquisition
CT scan was performed with 128-multidetector computed tomography scanner (SOMATOM Definition AS+, Siemens Healthcare, Forchheim, Germany). The following image acquisition parameters were used: peak voltage of 140 kVp, tube current adjusted by CARE Dose4D software (Siemens Medical Solutions, Erlangen, Germany), 128 × 0.67-mm detector collimation, 0.7 beam pitch, 0.5-s gantry rotation time, and reconstruction slice thickness of 0.6 mm using a bone kernel. Axial data were reconstructed with 0.6-mm-thick sections at 0.6-mm intervals for sagittal and coronal reformation.
Radiographic parameter measurements
Definition of bony cubital tunnel
Assuming that the cubital tunnel is a semi-circular structure with the line connecting the trochlear and the medial epicondyle as its axis, we could measure the actual nerve passing space. Typical craniocaudally directed axial CT images could not accurately characterize the cubital tunnel (Fig. 3). The bony cubital tunnel in this study was further defined in the following sections.
Anatomical landmark
The medial border of trochlear can be expressed as a fan-shaped plane of approximately 20 degrees varus to the axis of humerus and posterior slope of approximately 15 degrees for the axis of ulna (Fig. 4).
The center of the trochlear articular surface was defined as the center of the trochlear border (CTB), and the point of the medial epicondyle, which was the point farthest to the fan-shape, was designated as the apex of the medial epicondyle (AME). The line connecting AME and CTB was defined as the axis of bony cubital tunnel (ABT), which enabled the measurement of the characteristics of trochlear and medial epicondyle in the bony cubital tunnel. The articular surface of the medial trochlear border became the lateral wall while the medial epicondyle surface constitutes the medial wall (Fig. 5).
The roof, entrance, and exit of the bony cubital tunnel
The ceiling of the actual cubital tunnel is comprised of Osborne’s fascia; in this study, the plane that was formed from the line connecting the points on the lateral wall and medial wall, which constitute identical degree to ABT, was defined as the ceiling of the cubital tunnel. By taking the course of the ulnar nerve into account, the effect of the bony structure would almost disappear as the trochlear border passes the plane where the axes of the humerus and ulna are located. Therefore, the axis of the humerus and axis of the ulna in the trochlear border were defined as the entrance and exit, respectively. The angle formed by the entrance and exit was defined as the curvature angle (Fig. 6).
Bony cubital tunnel volume
Cubital tunnel volume (CTV) was measured by importing raw CT data into the Materialize Mimics 21.0 software (Materialize Interactive Medical Image Control System, Materialize, Leuven, Belgium), and 3D modeling according to the definition of bony cubital tunnel was performed.
Cross-sectional area and minimal cross-sectional area angle
Cross-sectional area (CSA) of the bony cubital tunnel (through the ABT plane) was measured at every 1° in a clockwise direction from the axis of the humerus, and the smallest cross-sectional angle measured was defined as the minimal cross sectional area angle (MCSA) (Fig. 7).
Cubital tunnel depth and cubital tunnel angle
In this study, we used the image obtained by reslicing at 1° intervals around the ABT in the measurement of cubital tunnel depth (CTD) and cubital tunnel angle (CTA), and we defined the resliced image as a rotatory image (CT syngo Post-Processing Suite software, version VE 36A).
CTD was defined as the length of the line connecting the deepest point of the groove of the ulnar nerve, which is vertical to the line connecting the most prominent part of the inward trochlea and the most prominent part of medial epicondyle (Fig. 8). CTD was measured from each rotary image; the minimum, maximum, and difference between minimum and maximum of CTD were compared as the range of CTA.
Moreover, CTA refers to the angle resulting from two lines drawn over the medial surface of the trochlea and the inferior border of the medial epicondyle at the deepest point of the cubital tunnel (Fig. 9). CTA was measured from each rotary image; the minimum and maximum CTA and the difference between them were identified. The maximum CTD and CTA values served as the ideal parameter value in cubital tunnel view and the minimum CTD and CTA values represented the narrowest part of the cubital tunnel. In addition, the difference between the minimum and maximum CTD and CTA values indirectly indicates the degree of change in CTD.
We quantitatively measured radiographic parameters by employing the picture archiving and communication system as our image analysis software (Maroview, version 5.4, Marotech, Seoul, Korea). Radiographic parameters was measured by manually delineating with a cursor, and images were evaluated in bone window (width, 2000 HU; level, 500 HU).
Intraobserver reliability
The measurements were evaluated by two experienced surgeons who were blinded to patient information. To reduce errors, measurements were obtained twice by each of the surgeons, and the average values were calculated. Intraobserver reliability was recorded using the criteria of Winer (degree of bias and mean squared error) [20]. Reliability was classified according to intraclass correlation coefficient as follows: absent to poor (0–0.24), low (0.25–0.49), fair to moderate (0.50–0.69), good (0.70–0.89), or excellent (0.90–1.0). We achieved an intraobserver reliability of 0.94. There were no missing data.
Statistical analysis
The measured CTV, CSA, MCSA, CTD, and CTA were presented as mean (range). Each radiographic parameter was analyzed using t test. Descriptive statistical analyses were performed using SPSS version 20.0 software (IBM Corporation, Armonk, NY), with an alpha level of 0.05.