This study was approved by the local institutional review board and informed consent was obtained from all enrolled patients (2018AS0050). The study protocol conformed to the ethical guidelines of the 1975 Declaration of Helsinki. Of the 212 wrists that had undergone USO for idiopathic UIS between June 2005 and March 2016, we selected 136 wrists (124 patients, right wrist: 68, left wrist: 68, both wrists: 12) that met our inclusion and exclusion criteria and reviewed them retrospectively. The inclusion criteria included patients with idiopathic UIS who underwent USO and postoperative follow-up for at least one year. Patients with a prior trauma history and/or comorbidities, such as diabetes, thyroid or parathyroid disease, renal disease, congenital metabolic bone disease, rheumatoid arthritis, severe obesity, and low bone density, and patients on glucocorticoid therapy, or with a smoking or alcohol consumption history less than one month before USO were excluded to avoid false conclusions on the osteotomy healing.
The diagnostic criteria for UIS included symptoms of ulna-sided wrist pain that aggravated with pronation and ulnar deviation of the wrist, pain during the ulnar stress test, tenderness in the ulnocarpal joint, positive ulnar variance in pronated-grip radiographs during either neutral rotation or cystic changes in the lunate or triquetrum, and degeneration of the lunate or triquetral cartilage in Magnetic Resonance Imaging.
To compare the osteotomy technique’s effect on parallel osteotomy and osteotomy healing, the wrists were divided into two groups according to the osteotomy technique (Group 1: freehand osteotomy, 74 wrists; Group 2: guided osteotomy, 62 wrists). The osteotomy reduction gap (the longest distance between two osteotomized surfaces after osteotomy reduction, Fig. 1a), the time to osteotomy union and the time to complete osteotomy consolidation were compared between the groups.
To identify the surgical factors affecting the time to osteotomy union and the time to complete osteotomy consolidation, we performed multiple regression tests for the direction of osteotomy, the gap after reduction osteotomy, length of ulnar shortening, usage of a lag screw, and plate type used for osteotomy fixation. Additionally, the cut-off length of the osteotomy reduction gap to obtain union within six months and the maximum period to achieve complete osteotomy consolidation were statistically calculated.
Surgery and postoperative rehabilitation
Distal diaphyseal USO was performed by two experienced surgeons under general anesthesia or brachial plexus block. Both surgeons used the same criteria to diagnose idiopathic UIS and the same surgical techniques. The periosteum of the ulna was not stripped during surgery to preserve the local blood supply around the osteotomy site. In group 1, freehand osteotomy was performed according to the previously described technique, and the osteotomy was fixed with a 3.5 mm limited contact-dynamic (LC-DCP; Dupuy-Synthes, Paoli, PA, USA) or locked (LCP; Dupuy-Synthes, Paoli, PA, USA) compression plate, after manual reduction. In group 2, guided osteotomy was performed with a 2.7 mm (Dupuy-Synthes, Paoli, PA, USA) or 3.5 mm (Acumed, Hillsboro, OR, USA) LCP-based ulna osteotomy system, the dedicated surgical system for guided USOs. All osteotomies were performed obliquely or transversely using an orthopedic oscillating saw around the distal one-third of the ulna, followed by plate fixation. To prevent heat injury to the osteotomized ulna during surgery, we stopped the sawing intermittently and cooled the osteotomy site by continuous cold saline irrigation in both groups . We prioritized maintaining the inborn longitudinal and rotational alignment of the ulna over minimizing the osteotomy reduction gap during reduction osteotomy. Even in a non-paralleled osteotomy, we performed reduction by moving the distal segment proximally along the longitudinal axis to avoid angulated or rotated ulna after reduction osteotomy. Before osteotomy, arthroscopies were performed for all the wrists to verify UIS and debride degenerated ulnocarpal structures [6, 17]. After surgery, both groups followed the same protocol for postoperative rehabilitation. Tolerable active wrist and forearm movements were allowed after placing the wrists in a short-arm splint for four weeks.
Background data collection
The patients’ background data, including sex, age, affected side, body mass index (BMI), and bone quality (second metacarpal cortical percentage: 2MCP), were recorded . BMI was calculated using the patients’ height and weight as BMI = weight [kg] / height [m]2 . Patients with severe obesity (BMI ≥ 30 kg/m2) were excluded from this study[19, 20]. For bone quality assessments, we calculated the 2MCP from true posterior-anterior views of the preoperative hand or wrist radiographs . All simple radiographs were taken on normal films (distance, 110 cm; power, 57 KV). The narrowest point (isthmus) of the second metacarpal shaft was focused and magnified to optimize visualization with the picture archiving and communication system (PiView STAR, Infinitt Healthcare Co. Ltd, Seoul, South Korea), a reliable method for orthopedic measurements [21, 22]. The transverse diameter of the isthmus of the second metacarpal bone was measured (A). Then, parallel measurements were made of the cancellous or intramedullary component at the same location (B) . We used the formula [(A - B) / A] × 100 to calculate the 2MCP (Fig. 2) . All variables were measured twice every two weeks by two independent orthopedists to avoid intra- or inter-observer errors, and the mean was calculated for each patient. Patients with 2 MCP < 50% were considered to have a higher likelihood of osteoporosis and were excluded from this study . Complications associated with osteotomy healing, such as metal failure, screw loosening, delayed union, nonunion, and refracture after plate removal, were determined.
Data collection on healing after osteotomy
We divided the healing process into osteotomy union and osteotomy consolidation. Osteotomy union was defined by a callus formation bridging all cortices of the four planes observed on simple radiographs and no pain at the osteotomy site in the manual stress test. Osteotomy consolidation was defined by the disappearance of any trace at the osteotomy site in all four planes of simple radiographs [15, 23]. A union achieved after six months was considered a delayed union; when no union occurs within six months and no radiographic improvements occur during three consecutive months, it was defined as a nonunion . Four plane simple radiographs (posteroanterior, lateral, internal, and external oblique views) were taken monthly until osteotomy union was achieved and every three months until osteotomy consolidation was completed to determine the length of the osteotomy reduction gap, the time to osteotomy union, and the time to complete consolidation for all the wrists. Two orthopaedists blindly determined these values, twice every two weeks, and the average for each wrist was calculated.
Data collection on surgical factors affecting the healing process
We recorded the direction of osteotomy, length of ulnar shortening, usage of a lag screw, and plate type used in each wrist to analyze the surgical factors affecting osteotomy healing. Based on the direction (an acute angle between the osteotomy plane and longitudinal axis of the ulna), the osteotomy was classified as transverse and oblique. The transverse osteotomy was defined by the angle ≥ 60° between the osteotomy plane and longitudinal axis, while oblique osteotomy was defined by the angle < 60° (Fig. 1b). The ulnar variance was measured using the method of perpendicular in a true posterior-anterior simple radiograph of the wrist . The length of ulnar shortening was calculated as the difference between the preoperative and immediate postoperative ulnar variance. The lag screw usage was recorded as a “yes” or “no”, and the plate type used for osteotomy fixation was recorded as either “LC-DCP” or “LCP”. The osteotomy direction and length of ulnar shortening were also blindly measured twice every two weeks by two orthopaedists and the average for each wrist was calculated.
Sample size and statistical analysis
Using data from the study by Gaspar et al., a prior power analysis was performed to determine the sample size needed to detect a difference in time to osteotomy union between freehand and guided osteotomies using the t-test and Pearson's chi-square test. Assuming a normal distribution and effect size of 1, we needed to enroll a minimum of 62 wrists in each group to detect a significant difference with 80% power (α = 0.05, β = 0.2). The sample size was calculated using the G*Power program (version 3.1.9).
To confirm that there were no differences in patients’ background characteristics between the two groups, a T-test was performed for age and BMI and Pearson's chi-square test for sex, affected side, direction of osteotomy, use of a lag screw, and type of plate used. The Mann-Whitney test was performed for 2MCP and length of ulnar shortening. To compare the time to osteotomy union and complete consolidation, and osteotomy reduction gap between the two groups, the Mann-Whitney test was also performed following the Bonferroni correction (α = 0.05/4 = 0.0125).
Multiple regression tests were performed to identify the surgical factors affecting healing. The factors, as described in the study design section, were considered as potential surgical factors associated with osteotomy healing. The cut-off length of the osteotomy reduction gap to obtain osteotomy union within six months and the maximal periods to achieving complete osteotomy consolidation were statistically calculated using receiver operating characteristic (ROC) curves. P < 0.05 was considered statistically significant in this regression test.