Keywords
cadaver - diaphyseal osteotomy - metaphyseal osteotomy - ulnar impaction syndrome - ulnar-shortening osteotomy
Ulnar impaction syndrome is a well-known cause of ulnar sided wrist pain stemming from increased load on the ulnocarpal side of the wrist. Numerous techniques have been described to reduce load on the carpus, but ulnar-shortening osteotomy has long been considered the standard of care.[1]
[2]
[3]
[4]
[5]
[6]
[7]
[8]
[9]
[10]
[11] Small changes in ulnar variance create significant changes in force transmission through the ulnocarpal joint.[12]
Classically, diaphyseal osteotomy has demonstrated encouraging short- and midterm results[13]
[14]
[15]
[16]
[17] but several complications have been reported with this technique, including residual ulnar positive variance,[17] hardware prominence requiring removal,[18]
[19]
[20]
[21] delayed union or nonunion,[22]
[23]
[24]
[25] and complex regional pain syndrome.[25]
[26] More recently, distal metaphyseal ulnar-shortening osteotomy has been reported in several studies with promising results.[27]
[28]
[29]
[30] Metaphyseal cancellous bone has richer blood supply and is less resistant to osteotomy compared with diaphyseal bone. This results in less bone necrosis from heating, thereby reducing the risk of nonunion and the time to return to unrestricted motion.[29]
[31]
[32] Biomechanical studies have reported significant reduction of the load across the ulnocarpal joint following metaphyseal osteotomies.[33] Additionally, osteotomy at the distal metaphysis does not interfere with the distal interosseous membrane,[34] which facilitates bony reduction and may allow greater shortening than diaphyseal osteotomy.
The purpose of this cadaveric study was to compare the biomechanical characteristics between diaphyseal and metaphyseal ulnar-shortening osteotomy with respect to (1) the maximal shortening achieved at each osteotomy site and (2) the force required to achieve shortening at each site as an indirect measurement of soft tissue constraint. We hypothesized that metaphyseal ulnar-shortening osteotomy (MUSO) allows for greater shortening compared with diaphyseal ulnar-shortening osteotomy (DUSO) with less stress on the soft tissues.
Methods
Specimens
Ten fresh frozen cadaver upper extremities were procured from the Anatomy Gifts Registry (Hanover, MD). This was a sample of convenience. The arms were intact from the scapula through the fingertips. The specimens included six left arms and four right arms from five male and five female donors, with a mean age at time of death of 71 years (range, 61–81 years). There was no history of trauma or surgery to any of the specimens. Prior to the experiment, ulnar variance of each specimen was measured on a posteroanterior radiograph that was obtained in neutral forearm rotation, 90 degrees of elbow flexion, and with the wrist in neutral position with respect to flexion–extension and radioulnar deviation.
Experimental Design
The experimental setup in neutral forearm rotation is illustrated in [Fig. 1]. A longitudinal incision along the ulna was carried down from the middiaphysis to the tip of the ulnar styloid. The forearm was secured to a wooden board by drilling a 5.0-mm Schanz's pin through the proximal ulna and into the board. The Schanz pin served as the static post for force measurements. A 20-mm transverse section of bone was resected with an oscillating saw at the metaphysis 20-mm proximal to the distal radioulnar joint (DRUJ). A ruler was used to ensure accuracy of the length of resection. Controlled ulnar shortenings in increments of 5 mm were performed until maximal shortening was achieved. Precut 5-mm acrylic glass blocks were used to ensure accuracy and consistency of the shortening procedure, for example, three of the blocks were placed at the osteotomy site to replicate 5 mm of shortening; two blocks were used to replicate 10 mm of shortening, etc. A Futek load cell (model LSB200, Futek, Irvine, CA) was attached to a distal post that was clamped to the surgical board and used to measure the force required for each sequential 5-mm of shortening (maximal capacity of 100 lbs and mean error of ± 0.1% of rated output). Proximally, a screw connected to the load cell was clamped to the distal ulnar metaphysis via lobster claw forceps. One of the investigators held steady tension during the shortening procedure to allow for recording of the forces to take place across 5 seconds. Ten measurements per second were recorded automatically on LabVIEW (National Instruments, Austin, TX), and the average of these recordings over 5 seconds was used as a single-data point. After the measurements were completed at the metaphysis, a 3.5-mm plate and 3.5-mm cortical screws (LC-DCP, DePuy Synthes, Paoli, PA) were used to fix the resected bone at the transverse osteotomy site to restore the anatomic length of the forearm.
Fig. 1 Experimental setup: a longitudinal incision along the ulna was carried down from the middiaphysis to the tip of the ulnar styloid. The forearm was secured to a wooden board with a 5.0-mm Schanz's pin proximally, which served as the static post for force measurements. A 20-mm section of bone was resected at the metaphysis just proximal to the DRUJ. Proximally, a screw connected to the load cell was clamped to the distal ulnar metaphysis via lobster claw forceps. DRUJ, distal radioulnar joint.
A transverse diaphyseal osteotomy in the same fashion was then performed at a distance approximately 10-mm proximal to the proximal end of the metaphyseal osteotomy. This corresponded to a point proximal to the proximal attachment site of the distal interosseous membrane as described by Arimitsu et al.[34] Measurement procedures were repeated for incremental shortenings of 5 mm. Force measurements of the diaphyseal osteotomy were performed under three conditions as follows: (1) with the interosseous membrane (IOM) intact, (2) after transection of the central band of the IOM, and (3) after extensive release of the IOM and muscular attachments to the diaphysis.
Statistical Methods
A priori power calculation based on a clinically significant difference of 2 mm of shortening and a large effect size (Cohen's d of 0.8) determined that a sample size of 10 for each group yields power of 0.50 which is comparable to other experimental cadaveric studies with similar statistical measures.[35]
[36] Cadaveric studies control variables better than population studies and often do not require large sample sizes to achieve normal distribution and normal variance for acceptable statistical analysis. One-way analysis of variance (ANOVA) was used to compare differences in tensile force for each incremental shortening. Repeated measures ANOVA was used to evaluate the effect of sequential soft tissue release on the diaphyseal shortening. Data are presented as mean ± standard error of the mean. Post hoc analysis using Bonferroni's correction was used to determine differences between each group if initial ANOVA demonstrated statistical significance. Pearson's correlation coefficient was used to determine the relationship between preexperimental ulnar variance and maximal shortening achieved. Statistical significance was set at p < 0.05.
Results
Data from one cadaver was corrupted due to a software malfunction and was therefore excluded from analysis. In the remaining nine specimens, mean ulnar variance was −0.2 mm (range, −1.7 to 1.2 mm). Maximal shortening (mean, 9.1 ± 0.7 mm) was achieved with MUSO which was significantly greater compared with the amount of shortening achieved with DUSO with intact interosseous membrane (6.9 ± 0.3 mm, p < 0.01) and DUSO after central band release (7.4 ± 0.4 mm, p < 0.05). Similar amount of shortening was achieved between MUSO and DUSO after extensive interosseous membrane and muscle release was performed (9.1 ± 0.7 vs. 8.6 ± 0.4 mm, p = 0.50).
The force required for maximal shortening was lower for MUSO (6.1 ± 0.7 lbs.) compared with DUSO (6.9 ± 0.8 lbs.) but this was not statistically significant. For DUSO, incrementally less force to achieve maximal shortening was recorded after central band release (6.6 ± 0.4 lbs.) and after extensive soft tissue release (6.4 ± 0.9 lbs.); however, this difference was not statistically significant.
Discussion
The results of our study support our hypothesis that MUSO allows for greater amount of shortening in comparison to diaphyseal osteotomy with similar tensile force required for maximal shortening for the two techniques. Increased shortening with diaphyseal osteotomy was only achieved after substantial release of the soft tissue attachments to the ulna. These findings are consistent with the results by Arimitsu et al[34] who demonstrated the contribution of the distal interosseous membrane to DRUJ stability. They noted the greater longitudinal resistance to shortening when the osteotomy was performed proximal to ulnar attachment to the distal interosseous membrane which may lead to difficult healing of the osteotomy site. The authors posited that if the purpose of the shortening procedure were to improve DRUJ stability, the optimal osteotomy site should be 60-mm proximal to the ulnar styloid to preserve the soft tissues; while patients without DRUJ instability may benefit from distal metaphyseal osteotomy to facilitate union.[34]
Biomechanical studies have demonstrated that diaphyseal osteotomy increased DRUJ stability at the expense of increased DRUJ joint reaction force.[34]
[37]
[38] Nishiwaki et al[37] demonstrated that 3 to 6 mm of shortening led to increase tension of the triangular fibrocartilage complex and increased DRUJ stability, provided that the radioulnar ligament is at least partially intact. Recent studies have further sought to delineate the optimal site for ulnar-shortening osteotomy, with various studies, citing the distal 25% of the ulna as the recommended position.[2]
[3]
[34]
[39]
[40]
[41]
The rate of nonunion after ulnar-shortening osteotomy varies between 0 and 11% in the literature which is often attributed to inadequate bone-to-bone contact and is more problematic in the diaphysis.[28] Therefore, ulnar-shortening osteotomy performed through metaphyseal bone has gained popularity, with proponents postulating that the cancellous bone at the distal metaphysis has richer blood supply and facilitates reduction of the osteotomy site, allowing for better bone-to-bone contact and higher potential for successful healing of the osteotomy.[21]
[27]
[28]
[31] Various techniques for MUSO have been described.[8]
[21]
[27]
[31]
[42] In a biomechanical study of the closing wedge osteotomy technique at the distal ulnar metaphysis, Greenberg et al[33] demonstrated significant reductions in ulnar load in both static and dynamic phases of wrist motion without significant increases in transverse force across the DRUJ. The mean shortening achieved by the investigators was 2.8 mm in seven cadaver arms.[33]
There is a paucity of data on the long-term clinical effects of MUSO on DRUJ stability, but functional outcomes have appeared promising. Compared with a traditional diaphyseal technique fixed by plate and screws, Sennwald et al[21] noted nearly equivocal outcomes with respect to pain, satisfaction, wrist range of motion (ROM), and grip strength but greater shortening, earlier time to union, less implant-related pain, and lower rate of implant removal using an oblique metaphyseal osteotomy fixed by two lag screws. Recent studies have demonstrated improved QuickDASH (disabilities of the arm, shoulder, and hand) score, visual analog pain score, and wrist ROM after transverse osteotomy at the distal ulnar metaphysis using a low-profile 2.0-mm locking compression distal ulna plate for ulnocarpal abutment syndrome after failed conservative management.[28]
[43] A comparison of MUSO (14 patients) and DUSO (21 patients) demonstrated greater ulnar shortening (4.8 vs. 3.4 mm), shorter tourniquet time (46 vs. 72 minutes), and greater postoperative improvement in pain and functional (QuickDASH) scores for the MUSO group.[30] The difference in shortening, however, was attributed to the MUSO group requiring greater shortening rather than the technique itself. Our cadaveric results demonstrated that an additional 4 mm of shortening (for a maximal of approximately 9 mm) may be achieved with transverse metaphyseal shortening if warranted clinically; however, it must be cautioned that the additional shortening would result in fixation between the epiphyseal and diaphyseal bone which squanders the metaphyseal healing potential.
Shortenings of this magnitude are rarely needed in the setting of idiopathic ulnar impaction syndrome but may be needed after distal radius fracture malunion with severe shortening or in the setting of distal radius growth arrest in the pediatric population. For a more clinically significant scenario, we demonstrated slightly lower force transmission with metaphyseal compared with diaphyseal shortening. We cannot attest as to why greater shortening was achieved at the metaphysis without a concomitant increase in force. We postulate that this may be attributed to differences in modulus of elasticity between the soft tissues at the DRUJ and the wrist compared with soft tissues at the diaphysis. This slight increase in force required to achieve shortening at the diaphysis may also account for some of the differences in clinical results seen in recent studies comparing the two techniques.[21]
[30]
There are several limitations to this study. First, the mean ulnar variance of our specimens was approximately neutral, with ulnar negative variance in a few specimens. The intended purpose of the shortening procedure is to treat ulnar impaction syndrome typically associated with ulnar positive variance[44]; therefore, the amount of maximal shortening may be even greater if specimens with positive variance were selectively tested. Second, we focused specifically on amount of shortening and the forces on the soft tissue necessary to achieve the desired shortening; therefore, it is difficult to extrapolate the effect of maximal shortening on ulnar load and DRUJ stiffness, although prior studies suggest improved DRUJ stability after shortening.[34]
[37]
[38] Third, all testing was performed with the forearm in neutral rotation; therefore, we were unable to elucidate the effect of excessive shortening on soft tissue tension with pronosupination. Lastly, testing of the diaphyseal osteotomy was performed after fixation of the resected metaphyseal osteotomy. Motion at the fixed ulna may have affected the outcomes; however, our results were consistent with previous biomechanical studies that highlighted the importance of soft tissue constraint on ulnar shortening.[34]
[37]
[38]
This study adds to the existing literature on ulnar shortening by demonstrating that metaphyseal ulnar osteotomy allows greater maximal shortening but requires similar force compared with diaphyseal osteotomy. With respect to clinical application, both sites of osteotomy can achieve sufficient shortening to decompress the ulnocarpal joint for most cases of ulnar impaction syndrome. The greater shortening from metaphyseal ulnar osteotomy may be reserved for severe cases of shortening, especially after distal radius malunion or in the setting of distal radius growth arrest in the pediatric population. Sequential release of the interosseous membrane permits increased shortening at the diaphysis, but requires extensive soft tissue release, which may not be compatible with clinical application.
