Evaluating Wire Configurations for Tension Band Constructs using a Canine Greater Trochanteric Osteotomy Model

Objective: To investigate the stability of four tension band wiring configurations alone


32
The tension band (TB) technique has been in use since the 1970s as a method to 33 resist distracting forces from a ligament or tendon on a bone fragment and convert those 34 tensile forces into compression to help stabilize the fragment. 1-3 TB has been used for 35 fixation in a variety of locations and clinical scenarios, including transpositions of the 36 tibial tuberosity and osteotomies or fractures of the medial malleolus, distal fibula, configuration was stronger than a single wire loop. 11,12 In an ulna osteotomy model, a 48 double loop configuration resisted greater load at 2 mm of displacement compared to the 49 standard figure-of-eight with two twists. 13 A dual interlocking single loop configuration 50 resisted similar loads to the standard figure-of-eight constructs. This study also showed 51 that the wire must be placed in contact with the pins and the bone fragment to prevent 52 fragment displacement with low loads. 53 Previous TB studies have incorporated a variety of configurations in the presence 54 of K-wires with or without other securing devices, such as lag screws or bone staples, to 55 determine the most effective (i.e., strongest, most stable) wire construct. [11][12][13][14][15][16][17][18][19][20][21][22] However, 56 the load resistance of the wire portion alone, without the confounding influence of K-wire 57 stabilization, has not been evaluated and may inform improved TB wire designs to 58 mitigate the persisting complications with TB fixation. While cerclage wires are known 59 to resist higher loads before loosening when tied to greater tension, 23 the tension achieved 60 with different tension band wire configurations is unknown. The objectives of this study 61 were to examine 1) the amount of initial tension generated during tying for various TB 62 configurations and 2) their ability to resist applied tensile loads before elongating to 63 failure. We hypothesized that, compared with figure-of-eight configurations with one or 64 two twists, dual interlocking single loop and double loop configurations would generate 65 greater initial tension, resist greater load to 2 mm of displacement, and better retain 66 residual tension with incremental cyclic loading. Additionally, we hypothesized that the 67 load required to cause 2 mm of displacement (in a monotonic test) and the remaining 68 residual tension after reaching 2 mm of displacement (in a cyclic test) would be 69 positively correlated with the initial tension generated during tying the wire.

72
A trochanteric osteotomy model was designed to evaluate the load resistance of 73 the wire portion of the construct independently, without contributions from the K-wires.

74
An anatomically correct, solid brass "femur" was milled using a computer model created 75 from a computed tomography scan of a normal right femur of a 30-kg canine. The solid 76 femur model was modified to simulate a greater trochanteric osteotomy by cutting the 77 trochanter fragment off the "bone" at an angle of 45 degrees to the long axis ( Figure 1).

78
Both cut surfaces were polished to minimize friction as the trochanter fragment moved 79 proximally when loaded. Two 2.4-mm diameter 316LVM stainless steel pins were 80 inserted perpendicular to the "osteotomy" line into the craniolateral and caudolateral 81 aspects of the trochanter fragment. Because the pins were only in the trochanter fragment 82 and did not extend across the osteotomy line into the rest of the "bone," the trochanter 83 fragment was only constrained by the wire configuration being assessed. To facilitate 84 creation of the TB loops, the exposed pin tips were bent over, and a 2.5-mm diameter 85 hole was drilled transversely through the distal aspect of the proximal metaphysis to 86 serve as the distal anchor for the TB wire. This hole was positioned 12 mm distal to the 87 distal edge of the osteotomy and 6 mm in from the lateral surface.

88
To test the TB constructs in the orientation of physiological loading, the femur 89 model was rigidly fixed to a custom jig at an angle of 45° and secured in a servohydraulic Everbuilt, Home Depot) was fitted at the fragment apex to mimic the attachment site of passed around the pins, with the loop beside the more cranial pin. The free end of the 117 distal wire was passed across the lateral aspect of the femur model and through the loop 118 of the proximal wire. The free end of the proximal wire was passed across the lateral 119 aspect of the femur model and through the loop of the distal wire. The two free ends were 120 introduced into a wire tightener (Item 391.21,DePuy Synthes Vet,West Chester,PA) 121 and secured to the cranks. The cranks were turned simultaneously to tighten the wires 122 until the operator deemed appropriate tightness had been reached. While holding that 123 tension, the wire tightener was twisted to lock the ends to each other.

124
The DL configuration was formed from a length of wire with a 2-mm loop formed 125 at its midpoint ( Figure 2D). This loop was positioned between the pins and the free ends 126 passed around the pins, crossed over the lateral aspect of the femur model, passed 127 through the distal anchor hole, and bought back, and through, the loop. The ends were 128 introduced into a wire tightener, secured to the cranks, and the wires tightened 129 simultaneously until the operator deemed appropriate tightness had been obtained. While 130 maintaining crank tension, the wire tightener was bent over. The cranks were turned in 131 reverse to release 1 cm of wire, which was bent flat and then cut.

134
For each test, the trochanter fragment was moved to a "reduced" position and 135 secured with the temporary pin, and the load cell output was set to zero. The wire 136 constructs were positioned, but not tightened, and the temporary pin was removed. The 137 wire configurations were then tightened and the knots completed. To examine the 138 effectiveness of the twist-and-lay technique used for the OT and TT constructs, the tension generated during the tying process (tying tension) was recorded for these two 140 configurations prior to completing the final folding or setting of the knot. The initial 141 tension was recorded as the initial force after tying was complete but before additional 142 loads were applied.  (cycle) was treated as an R-side random effect, and the covariance matrix was modeled 184 with a first-order autoregressive, first-order moving average structure (ARMA(1,1)). For each cycle, the residual tensions were compared between configurations using simple 186 effect differences based on least squares means with Tukey-Kramer adjustments for 187 multiple comparisons. The relationship between the final residual tension and initial 188 tension was determined with linear correlation analysis, and the Pearson correlation 189 coefficient was calculated.

191
During the monotonic tests, none of the wire configurations broke before 2 mm of 192 displacement. By visual inspection, the DL initially underwent slight wire stretch prior to 193 the two bent-over wires lifting up as load continued to be applied. The DISL initially 194 elongated via stretching of each end loop. For both figure-of-eight constructs, after a brief 195 period of initial stretch or flattening, elongation occurred by untwisting of the knot(s).

196
The twist-and-lay method of tying was effective for the TT configuration,197 increasing the tension by an average of 39% after bending over the twisted wire (42 ± 6 198 N tying tension vs. 56 ± 12 N initial tension, p = 0.020). The tension in the OT construct 199 did not change significantly with the twist-and-lay method (33 ± 5 N vs. 36 ± 9 N, p = 200 0.38). Greater initial tension was generated with the DL configuration (128 ± 24 N) than 201 the DISL (46 ± 5 N), TT (56 ± 12 N) and OT (36 ± 9 N) configurations (p < 0.0001 for 202 all, Figure 3). The initial tension of the DISL was not significantly different than that of 203 the TT or OT, but it was greater in the TT than in the OT (p = 0.041). The failure load at 204 2 mm of displacement was greater for the DL (402 ± 39 N) than the DISL (206 ± 14 N),

205
TT (199 ± 20 N), and OT (165 ± 15 N) configurations (p < 0.0001 for all, Figure 4). The 206 failure load of the DISL was not significantly different than that of the TT configuration, 207 but both DISL and TT failure load was greater than that of the OT configuration (p = 208 0.0092 and 0.039, respectively). The failure load was linearly correlated with the initial 209 tension (slope = 2.3 ± 0.15, r = 0.945, p < 0.0001).

210
For the incremental cyclic tests, the modes of failure were the same as described OT (217 ± 30 N, p < 0.0001) constructs (Figure 6). At failure, which occurred at different 220 cycles for each sample, the residual tension after the last cycle was significantly higher in 221 the DL (11 ± 3 N) than in the DISL (1.0 ± 1.9 N), TT (2.7 ± 3.7 N), and OT (1.4 ± 3.5 N) 222 configuration (p < 0.0001 for all, Figure 7). The final residual tension after reaching 2 223 mm was only greater than zero for DL and was not significantly different from zero for 224 DISL, TT, and OT. The residual tension after the last cycle was linearly correlated with 225 the initial tension before the first cycle (slope = 0.11 ± 0.019, r = 0.736, p < 0.0001).

227
As hypothesized, independent of K-wire stabilization, the double loop tension 228 band wiring construct performed best of the four configurations tested, generating the 229 greatest initial tension (2.3-3.5 times greater than the other configurations), resisting the 230 highest load before 2 mm of displacement (2.0-2.4 times greater), and maintaining the 231 greatest percentage of the initial tension under incremental cyclic loading. The failure 232 load was highly correlated with and scaled with the initial tension by a factor of 2.3, so 233 creating more tension in a TB construct during tying means that it will resist greater loads 234 before it begins to loosen. Our results are consistent with previous studies, where the 235 double loop generated greater static tension and a higher yield load 26 and resisted higher 236 loads at 2 mm of displacement, 13 when compared to single loop or twist knots. In our 237 study, the figure-of-eight TB construct with two twists resisted distraction of the 238 fragment better than the one with one twist and thus was able to achieve a 21% higher 239 force before failure at 2 mm (199 N vs. 165 N). Similarly, in a human cadaver study 240 involving reduction of an olecranon osteotomy, the figure-of-eight TB wiring using two 241 tightening knots was more effective in preventing motion under forces involved in active 242 mobilization of the elbow immediately after operation compared to one with one twist. 14

243
The greater initial tension with the DL results primarily from the use of a wire 244 tightener to tighten the construct. The cranks enable the wire to be tensioned more 245 effectively than other gripping instruments, and while some of that tension is lost as the 246 arms are folded over, much is retained, resulting in the higher loads measured. The ability 247 to resist load in these constructs relates to the mode by which the knots are "undone," arms, the DL configuration can better resist distraction, and higher loads must be applied 250 before it loosens to the point of failure. Given that the knot or fold is the weak point of 251 the system, this finding also suggests that more force is required to unbend two arms 252 (DL) than to untwist two wires wrapped around each other as is present in the other three 253 constructs. We found the following factors were important for producing the most secure

259
Contrary to our hypothesis, the DISL construct did not achieve a higher initial 260 tension than the TT or OT constructs, although it did resist a higher monotonic load 261 before 2 mm of displacement than OT, and it had a higher residual tension under cyclic 262 loading than OT in the early cycles. Tightening the more complex DISL configuration is 263 more difficult than the other configurations in this study, even with the use of the wire 264 tightener, which may explain the inability to achieve a higher initial tension than with the 265 figure-of-eight constructs. When loaded, the loops in the DISL elongated first, resulting 266 in yield and elongation of the construct at lower loads. After this initial elongation, 267 however, the configuration was stiffer and resisted higher loads, although this generally 268 occurred after the clinical failure point of 2 mm displacement. Therefore, using clinically 269 relevant criteria, the DISL construct was no more effective than the TT or OT constructs 270 and was more difficult to form and tie.
One unique aspect of this study is the analysis of the tying process to help 272 understand the differences between the different wire configurations in terms of the 273 separate tightening and securing processes during tying. The twist-and-lay technique used 274 for the figure-of-eight constructs was able to maintain (OT) or even increase by 39% 275 (TT) the wire tension during the flattening process. After the twist was formed to a 276 perceived tightness of just less than optimal, the knot was pulled up and an additional half

285
The femur model and setup for testing the TB configurations in this study were 286 designed to be anatomically correct and simulate a physiological loading direction on the 287 greater trochanter during gait, yet also isolate the performance of the wiring portion of 288 the construct and remove the contributions of K-wire stabilization and bone stiffness. In 289 previous studies using bone models, the materials used to simulate bone were more 290 compliant, such as Delrin® bar models or wooden patella models, 11,12,20,21 introducing 291 compliance within the testing structure that could potentially confound the mechanical 292 property measurements of the constructs alone. Similar to our study, some previous 293 studies used anatomical loading, but they inserted K-wires across the fracture gap for stability and thus did not isolate the performance of the wiring configurations under 295 loading. [11][12][13][14]16,[18][19][20][21][22]28,29 Other studies isolated the wiring configurations but did not 296 perform anatomical loading. 23,[30][31][32][33] To our knowledge our study is the first to incorporate 297 all of these aspects, providing an isolated comparison of various tension band wiring 298 configurations without contributions from other parts of the stabilization system.

299
In conclusion, our greater trochanteric osteotomy model, without K-wires 300 bridging the "fracture" gap, provided an effective means for isolating the performance of 301 different tension band wiring configurations. Testing all wires on the same "bone" 302 allowed for direct comparisons among the different configurations. The double loop was 303 the best configuration in this study, primarily due to its substantially higher initial 304 tension, which conferred the greatest resistance to load compared with the other three