The management of unstable distal tibia fractures remains challenging (23,24). Although IMN fixation of tibia shaft fractures is considered the “gold standard”, there is considerable concern about the use of IMN for distal metaphyseal tibia fractures. This apprehension is due to the problems associated with the limited availability of bone for distal fragment fixation, maintaining fracture reduction, and the risk of nail penetration into the ankle joint. These issues can frequently result in distal tibia fracture malunions, delayed unions, or nonunions (8,25). Although IMN surgical technique modifications (blocking screws, larger nails, oblique distal interlocking screws orientation, supplemental plate augmentation) were developed to address some of these concerns, very distal tibia fracture fixation problems continue (8).
The Expert tibia nail with ASLS capability is a novel concept that can convert the standard nail-interlocking screw to a fixed-angle construct for enhanced distal fragment fixation. Previous biomechanical studies using ASLS (18,20,26,27) have demonstrated an increase in both axial and/or torsional stability. Horn et al. (18) reported a twofold increase in axial stiffness between ASLS versus conventional screws (620 N/mm vs 1420 N, respectively). However, the mean age of the human cadaver specimens used in that study was 77 years. In addition, 10 mm diameter nails were utilized in each specimen regardless of the size of the cadaver tibia diameter. Although the ASLS axial stiffness values in the present study of composite bone tibiae were similar to those previously reported (18), axial stiffness in the conventional screw group were found to be higher and more comparable to the ASLS group. This suggests that ASLS, similar to conventional locking plate designs, may have its most substantial effect in the older and/or more osteoporotic patient. The Horn at al. study (18), however, did not assess the torsional merits of ASLS.
Gueorgueiv et al. (20), in a cadaver tibia model, similarly demonstrated that the torsional stability of three conventional distal interlocking screws was comparable to two ASLS screws. Wähnert et al. (27), compared a three-screw configuration with ASLS versus conventional screws in porcine bone and found that the ASLS group exhibited only a 10% increase in axial stiffness, but a 70% increase in torsional stiffness.
Our study suggests that there is no statistical difference (p>0.05) between any of the groups when two distal interlocking screws are employed regardless of screw orientation or the presence of ASLS. While the two-screw ASLS stiffness had trend to higher values in both axial and torsion loading, this was not statistically significant compared to other two-screw fixation. These results, similar to those of Wähnert et al. (27), demonstrated an approximate 10% increase in axial stiffness for ASLS versus conventional two-screw configuration; however, no difference in stiffness with respect to torsion was observed. In addition, the orthogonal conventional screws (Group 2) were also higher in torsional stiffness compared to the medial-lateral conventional screws (Group 1). In fact, the orthogonal screw model torsional stiffness was comparable to that of all of ASLS groups.
Our study also demonstrated that the axial and torsional stability of one-screw ASLS (Group 5) is not statistically different (p=0.99) from that of either of the two conventional screw constructs (Groups 1 and 2). The weakest group was the single conventional screw (Group 3), which showed similar axial loading stiffness but significantly (p<0.048) less torsion compared to the other groups. This suggests that a single conventional screw, while providing sufficient axial stiffness to the construct, may not provide sufficient torsional resistance, and therefore two conventional screws would be recommended for optimal IMN fixation. The single-screw ASLS group (Group 5) was statistically stiffer in torsion than the single conventional screw (Group 3), and had comparable axial stiffness and displacement to the two screw configurations in Groups 1, 2, and 4. This suggests that one ASLS may offer the same biomechanical characteristics as two conventional screw configurations, and may avoid the placement of multiple distal screws. Previous studies have documented that the placement of distal interlocking screws is not entirely benign, and in addition to the surgical time, radiation exposure, and costs, there is always the possibility of soft tissue injury during the procedure (28,29). The surgeon’s ability to maintain optimal construct stability while eliminating the risks associated with multiple screws constitutes a clear design advantage of the ASLS over more conventional locking screw designs in intramedullary nailing of the distal metaphyseal tibia fractures.
There are several study limitations. The tibia fixation and loading characteristics of synthetic bone utilized may not reflect the variations in bone quality and conformation encountered in vivo. However, because the composites are manufactured to achieve consistency in composition and configuration, direct comparisons across study groups involving different fixations are more feasible. Another limitation is the limited number of specimens tested per group; however, consistent and important relationships and differences in the groups tested were demonstrated. Finally, without repetitive cyclic loading, this study could not assess the effects of short- and long-term degradation of the biodegradable sleeve and/or conventional interlocking construct stability.
This is the first study to compare the biomechanical effects of IMN interlocking screw number and orientation between ASLS versus conventional interlocking screw constructs in a distal tibia fracture model. The study concludes that in this composite tibia model, one ASLS provides adequate axial and torsional stiffness and displacement compared to other two-screw configurations (both ASLS or conventional). Although two ASLS screws provide increased axial and torsional stiffness and less displacement compared to all of the IMN interlocking constructs, this difference was not statistically significant when compared to other two-screw conventional configurations. Finally, this biomechanical study demonstrated no statistically significant difference in the orthogonal versus parallel conventional interlocking screw placement in distal tibia fracture IMN fixation.
All nails and screw systems were provided by Synthes, the study was funded by the E. Burke Evans Research Foundation for Orthopedic Education. The authors thank Suzanne Simpson for editorial assistance with the manuscript.
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