Janne T. Nurmi

Porcine Tibia Is a Poor Substitute for Human Cadaver Tibia for Evaluating Interference Screw Fixation

Background Animal tissues are commonly used in anterior cruciate ligament graft fixation studies. Hypothesis Porcine bones and tendons provide good surrogates for human cadaver tissues in the biomechanical evaluation of anterior cruciate ligament reconstruction. Study Design Randomized experimental study. Method Three different tissue models—pure porcine (porcine graft fixed in porcine tibia, group 1), combination (human hamstring graft and porcine tibia, group 2), and pure human (human graft and tibia, group 3)—were compared using both cyclicloading and subsequent single-cycle load-to-failure tests to assess the effect of graft and bone tissue source (porcine vs human) on the fixation strength of anterior cruciate ligament reconstruction. Results In the cyclic-loading test, the displacement (slippage) after 1500 cycles was 2.0 mm ± 0.7 mm, 1.6 mm ± 0.4 mm, and 4.4 mm ± 1.9 mm for groups 1, 2, and 3, respectively (P< .001 between 1 and 2 vs 3). In the subsequent single-cycle load-tofailure test, the corresponding average yield loads were 668 N ± 157 N, 962 N ± 238 N, and 448 N ± 98 N, all differences being statistically significant. Conclusions In comparison to young human cadaver tibia, porcine tibia underestimate graft slippage and overestimate the failure load of the soft tissue graft in anterior cruciate ligament reconstruction. Clinical Relevance Porcine tibia does not provide a reasonable surrogate for human cadaver tibia for evaluating ACL reconstructions.

Interference Screw Fixation of Soft Tissue Grafts in Anterior Cruciate Ligament Reconstruction: Part 2 Effect of Preconditioning on Graft Tension During and After Screw Insertion

Background Preconditioning of tendon grafts is believed to eliminate natural viscoelasticity of the tendons and prevent knee laxity after anterior cruciate ligament (ACL) reconstruction. Hypothesis Preconditioned ACL grafts maintain their initially set tension. Study Design Randomized experimental study. Methods Forty-two human anterior tibialis (AT) tendon grafts were subjected to either no preconditioning (group 1), cyclic preconditioning (group 2), or isometric preconditioning (group 3). The residual graft tension was then recorded immediately after the application of an initial graft tension of 80 N and fixation into tibia with an interference screw, as well as 10 minutes later. In another experiment, the residual graft tension was recorded 1, 10, and 60 minutes after 10 AT nd quadrupled hamstring tendon (HT) grafts alone (no fixation) had been subjected to isometric preconditioning (80 N). Results Immediately after screw insertion, the residual (AT) graft tensions were 79 ± 19 N, 100 ± 17 N, and 102 ± 15 N in groups 1 through 3, respectively. Ten minutes later, the corresponding values were 49 ± 16 N, 60 ± 11 N, and 64 ± 12 N. For the AT and HT grafts alone, the residual graft tensions were 67 ± 2 N and 67 ± 2 N, 45 ± 2 N and 46 ± 4 N, and 29 ± 3 N and 34 ± 5 N at 1, 10, and 60 minutes, respectively. Conclusions A steady decrease (–60% within 60 minutes after initial tensioning) occurs in the initially set tension of the soft tissue ACL grafts. Clinical Relevance Clinically applicable preconditioning protocols cannot eliminate the intrinsic viscoelasticity from ACL soft tissue grafts, and thus, the reasonableness of preconditioning per se is questioned in ACL reconstruction.

Interference screw fixation of soft tissue grafts in anterior cruciate ligament reconstruction: part 1: effect of tunnel compaction by serial dilators versus extraction drilling on the initial fixation strength.

Background Compaction of the bone-tunnel walls by serial dilation is believed to enhance the interference screw fixation strength of the soft tissue grafts in anterior cruciate ligament (ACL) reconstruction. Hypothesis Serial dilation enhances the fixation strength of soft tissue grafts in ACL reconstruction over extraction drilling. Study Design Randomized experimental study. Methods Initial fixation strength of the doubled anterior tibialis tendon grafts (fixed with a bioabsorbable interference screw) was assessed in 21 pairs of human cadaver tibiae with either serially dilated or extraction-drilled bone tunnels. The specimens were subjected to a cyclic-loading test, and those surviving were then tested using the single-cycle load-to-failure test. Results During the cyclic-loading test, there were 3 fixation failures in the serially dilated and 6 failures in the extraction-drilled specimens but no significant stiffness or displacement differences between the groups. In the subsequent load-to-failure test, the average yield loads were 473 ± 110 N and 480 ± 115 N for the 2 groups respectively (P= .97) and no difference with regard to stiffness or mode of failure. Conclusions Serial dilation does not increase the strength of interference fixation of soft tissue grafts in ACL reconstruction over extraction drilling. Clinical Relevance The results of this experiment do not support the use of serial dilators in ACL reconstruction.

Bone density and insertion torque as predictors of anterior cruciate ligament graft fixation strength.

Background Bone mineral density and interference screw insertion torque are thought to be useful predictors of anterior cruciate ligament graft fixation strength. Hypothesis Bone mineral density and insertion torque are reliable predictors of anterior cruciate ligament graft fixation strength. Study Design Randomized experimental study. Methods The volumetric bone mineral density, maximum insertion torque, and initial fixation strength (determined both as cyclic-loading-induced displacement and yield load) were recorded on 21 pairs of anterior cruciate ligament reconstructions. To assess the accuracy of bone mineral density and insertion torque in predicting the fixation strength, half of the specimens were first used to determine the regression equations between the variables, and then the data from the remaining specimens were used to validate the prediction equations. Results Despite the relatively high group correlations, the ability of bone mineral density and insertion torque to predict the strength of graft fixation was poor. Errors ranging from –150% to 92%, –22% to 50%, –56% to 121%, and –23% to 50% were observed when bone mineral density and insertion torque were used to predict individual cyclic-loading-induced displacement and yield load, respectively. Conclusions Neither bone mineral density nor insertion torque provides a sufficiently accurate prediction of the fixation strength of an individual soft tissue anterior cruciate ligament graft. Clinical Relevance Bone mineral density and insertion torque cannot be used to estimate the strength of interference screw fixation in anterior cruciate ligament reconstruction.

Compaction Drilling Does Not Increase the Initial Fixation Strength of the Hamstring Tendon Graft in Anterior Cruciate Ligament Reconstruction in a Cadaver Model

Background Compaction of the bone tunnel walls has been proposed to increase the fixation strength of soft tissue grafts fixed with an interference screw in anterior cruciate ligament reconstructions. Hypothesis Compaction drilling does not increase the initial fixation strength of the hamstring tendon graft in comparison with conventional extraction drilling. Study Design Randomized experimental study. Methods Initial fixation strength of quadrupled hamstring tendon grafts fixed with bioabsorbable interference screws was assessed in 22 pairs of human cadaveric tibiae. Bone tunnels were drilled with either a compaction drill or a conventional extraction drill. Specimens underwent a cyclic-loading test and the surviving specimens were then loaded to failure in a single-cycle load-to-failure test. Trabecular bone mineral density at the site corresponding to the actual site of the tibial bone tunnel was determined by using peripheral quantitative computed tomography. Results During the cyclic-loading test, no significant stiffness or displacement differences were observed between the two drilling techniques. Three specimens failed in the compaction-drilling group, whereas there were no failures in the extraction-drilling group. In the subsequent single-cycle load-to-failure test, no significant differences between the two drilling techniques were found with regard to displacement at yield load, stiffness, or mode of failure. There was no significant difference in trabecular bone mineral density between the two groups. Conclusions Compaction drilling does not increase the initial fixation strength of the hamstring tendon graft compared with conventional extraction drilling.

A retrospective follow-up of ankle fracture patients treated with a biodegradable plate and screws

BACKGROUND Biodegradable fixation implants have been developed to avoid secondary hardware removal. The aim of this study was to retrospectively follow-up ankle fracture patients treated with a biodegradable plate and screws, and to evaluate the clinical outcome and occurrence of complications. METHODS Fifty-seven ankle fracture patients treated with biodegradable implants were invited, and a total of 50 were available to participate in this study. The follow-up included a review of each patients medical records, evaluation of radiographs, fracture reduction classification, and functional scoring. RESULTS There were 36 lateral malleolar and 14 bimalleolar fractures. No perioperative complications occurred. Average follow-up time was 17 months. All fractures healed. Fracture alignment was classified as anatomical in 49 patients and good in 1 case. The mean Olerud and Molander functional ankle score at final follow-up was 86. Eight patients had postoperative complications. These included delayed wound healing in 1 case, 3 cases of deep-vein thrombosis, and 4 soft tissue reactions. CONCLUSIONS According to the results of this retrospective study, the biodegradable implants used yielded fracture healing and functional results comparable to those previously reported after conventional metal fixation.

Biomechanical in vitro evaluation of the effect of cyclic loading on the postoperative fixation stability and degradation of a biodegradable ankle plate

The effect of cyclic loading on the postoperative fixation stability of a biodegradable ankle plate was tested biomechanically during 12 weeks of hydrolytic degradation. Fracture of the lateral malleolus was simulated, and the parameters of cyclic loading were chosen to represent the physiological conditions during the healing period. Additionally, the effect of cyclic loading on degradation was investigated by measuring the inherent viscosities. In Group I, the cyclic loading was conducted in four phases with gradual increases in estimated walking distance and speed during the healing period. In Group II, cyclic loading was conducted after 12 weeks. Group III was used as a control for inherent viscosity measurements. None of the specimens failed under cyclic loading. No significant differences were found between the loaded groups in any of the parameters measured. Additionally, no significant difference was found in inherent viscosities at 12 weeks. The initial fixation stability provided by the biodegradable ankle plate remains biomechanically unchanged over 12 weeks. Cyclic loading, applied either during or after 12 weeks of hydrolytic degradation, does not seem to have any clinically relevant effect on the fixation stability or the degradation properties.

Pullout strength of a biodegradable free form osteosynthesis plate

The Inion(®) Free Form Plate is a newly designed biodegradable plate. After drilling through the plate and tapping, a biodegradable screw can be inserted, followed by removal of the screw head. As an alternative a countersink screw can be used. Aim of the study was to compare the mechanical properties of the 1.4 mm Free Form Plate with the 2.0 mm conventional shaped plate. Mechanical testing of the plate pullout strength was conducted for the Inion(®) Free Form Plate fixed with an Inion OTPS™ 2.0 × 20 mm Screw. In addition, the failure mode was reported. Overlapping confidence levels were found with regard to the yield load, first peak load and maximum load, when comparing the Free Form Plate and the conventional 4-hole plate. The Free Form Plate fixed with a screw with head and countersink showed the highest stability at maximum load. The results of the mechanical stability testing showed no significant differences between the tested plates. The main failure mode was a failure of the screw shaft. The results of the current investigation imply that the 1.4 mm Free Form Plate could be used as an alternative to the 2.0 mm conventional shaped plate.

Bioabsorbable Versus Metal Screw in the Fixation of Tibial Tubercle Transfer: A Cadaveric Biomechanical Study

Background: In tibial tubercle transfer (TTT) procedures, the osteotomized and transferred tibial tubercle is usually fixed into the host bone using metal screws. Purpose: To compare the strength of fixation provided by a single bioabsorbable screw versus a metal screw for TTT. Study Design: Controlled laboratory study. Methods: Twenty-two pairs of human cadaveric tibiae were used to compare the fixation strength of a single 4.5-mm bicortical bioabsorbable or metal screw for TTT. In our 2-phase biomechanical testing protocol, the specimens were first subjected to a cyclic-loading test (1500 loading cycles between 50 and 300 N at 0.5 Hz frequency), after which they were loaded to failure (single-cycle load-to-failure test). To control for possible differences in bone quality, volumetric bone mineral density was determined using peripheral quantitative computed tomography. Results: No significant displacement differences were observed between the 2 groups for the cyclic-loading test. In the subsequent single-cycle load-to-failure test, the mean yield load was 566 ± 234 N in the bioabsorbable screw group and 984 ± 630 N in the metal screw group (P = .002). The failure mode of bioabsorbable screws was breakage and/or bending, and that of metal screws was bending and/or pull-out. Bone density was similar in the 2 groups. Conclusion: A metal screw seems to provide greater fixation strength than a biodegradable screw in the TTT of a human cadaveric knee. However, considering the maximum quadriceps pull in vivo, the strength of fixation provided by a biodegradable screw seems clinically sufficient. Clinical Relevance: Bioabsorbable screws, particularly if used in duplicate, could provide a viable option for metal screws in TTT fixation.