OTA 1999 Posters

The Role of Frame Symmetry in Hybrid External Fixation of the Proximal Tibia

Dennis J. Beck, Jr., MD; Michael J. Voor, PhD; Craig S. Roberts, MD; Department of Orthopaedic Surgery, School of Medicine; University of Louisville, Louisville, Kentucky

Purpose: Various biomechanical studies suggest hybrid fixators are less stable than other forms of external fixation. The question is whether the instability arises from the disparity between using rings and tensioned wires opposite half-pins or simply from the typically asymmetric design of the unilateral hybrid frame. The purpose of this study is to determine the influence of increasing frame stability while holding the ring/wire and half-pin arrangements constant. We hypothesize that improving the overall symmetry of the ring to frame connection reduces fracture site axial, shear and angular motion.

Methods: Hybrid fixation of an unstable extra-articular proximal tibia fracture (OTA Classification 41-A3.3) was modeled. A wooden dowel of 32mm diameter simulated the distal fragment and a 7 cm square wooden block simulated the proximal fragment. Two 1.8 mm K-wires were crossed at approximately 60 degrees to simulate transfixion of the zone immediately distal to the tibial plateau. The wires were tensioned to 110 kg across a 160- mm ring (Ilizarov, Smith & Nephew, Memphis, TN) that was in turn connected to a second identical ring. Both rings were fixed to a unilateral bar (Hex-Fix, Smith & Nephew Richards, Memphis, TN) anterior and parallel to the tibial model. Distal to the fracture site, three 5-mm half-pins were placed in the dowel and clamped to the bar. The half-pins were evenly spaced over 15 cm with the middle pin on the opposite side of the bar from the other two. An interfragmentary motion device (IMD) was mounted on the model above and below the fracture gap. The device simultaneously measures axial and lateral translation and angulation between the proximal and distal fragments. The IMD was mounted to measure two-dimensional motion in both the sagittal and frontal planes. Loading was applied in the Bionix 858 materials testing system (MTS, Minneapolis, MN) directly to the top center of the proximal block to simulate tibial plateau loading. The fracture site motion was recorded and compared at 100 N loading between each of three frame constructs: A) standard hybrid (no supplemental support), B) strut A (ring to bar buttress clamp connected between first and second half-pin), C) strut B (ring to bar buttress clamp connected between second and third half-pin).

Due to limitations in the dowel model, it was necessary to also model hybrid fixation of an extra-articular proximal tibia fracture (OTA Classification 41-A3.3) using a composite fiberglass tibia (Sawbones, Pacific Research <

Laboratories, Vashon, WA). The same hybrid constructs were tested as above including a box hybrid (additional two-ring group mounted to bar below all half-pins and connected to proximal rings using two 20 cm secondary Hex-Fix bars each at 150 degrees from the primary Hex-Fix bar). In these experiments, each specimen was tested in four loading modes based on position of load transmission to the tibial plateau (directly along the tibial axis, four cm medial, four cm posterior and four cm medial and posterior). Force transmission through the three fixator constructs was measured by a load cell on which the specimens were placed. The displacement of the MTS actuator was recorded for each test at 100 N load. Single factor ANOVA and post-hoc t-tests were applied to compare the actuator motion between groups.

Table 1. Fracture Site Motion at 100 N Axial Load (mean ± S.D.)

	Standard Hybrid	Strut A	Strut B
Axial Motion (mm)	3.41± 0.164	3.45 ± 1.231	2.08 ± 0.107
A-P Angulation (°)	0.59 ± 0.023	0.55 ± 0.101	0.42 ± 0.030
A-P Shear (mm)	0.29 ± 0.018	0.19 ± 0.043	0.14 ± 0.005

Table 2. MTS Actuator Motion (mm) at 100 N Axial Load (mean ± S.D.)

Loading Mode	Standard Hybrid	Strut B	Box Hybrid
Axial	2.70 ± 0.19	2.23 ± 0.01	1.98 ± 0.02
Medial	3.09 ± 0.14	2.75 ± 0.15	2.45 ± 0.02
Posterior	8.70 ± 0.04	8.81 ± 0.58	7.75 ± 0.04
Posteromedial	9.44 ± 0.09	9.98 ± 0.11	8.57 ± 0.19

Results: Statistically significant differences (p 0.05) were found between groups for all motions measured. The most fracture-site motion (Table 1) and proximal fragment displacement (Table 2) occurred with the standard hybrid construct. The strut constructs increased the stability over that of the standard hybrid, while the greatest stability was found with the box hybrid.

Discussion and Conclusions: The complete motion of hybrid frame constructs results from the inherent motion allowed by a ring and two crossed K-wires superimposed on the motion allowed by deformation of the frame with minimal contribution from the bending of the three half-pins. The design of this study is intended to keep the half-pin bending and the inherent ring/wire motion consistent between the four constructs tested. Thus our results demonstrate the direct improvement in fracture fixation obtained from simply enhancing the stability of the frame. As expected, the standard hybrid with no enhanced support allows the most motion. As frame symmetry improves and allows increasingly balanced load transmission from the proximal ring to the distal fragment, the fracture site motion decreases.

Because we restricted the loading to the tibial axis in the first set of tests, the results presented in Table 1 demonstrate the inherent fracture-site motion allowed by hybrid external fixator asymmetry. Efforts such as the strut supports tested here improve, but do not eliminate the influence of frame asymmetry. The second set of tests (Table 2) were intended to address more biomechanically relevant loading conditions. These include superimposed posterior and medial bending caused by muscle forces and the medial line of action of the mechanical axis of the lower extremity. Interestingly, the improvement in proximal fragment stability provided by the box hybrid is limited by the ability of the proximal wires to resist eccentric loading. As the loading became more eccentric the percentage of improvement decreased from 26% (axial) to 9% (posteromedial).

Standard hybrid fixators using only rings and wires opposite a unilateral bar and half-pins are inherently unstable due to the ring/wire flexibility superimposed on the asymmetric bending of the frame. By simply augmenting the frame itself to make it more symmetric, hybrids can be made biomechanically similar to historically proven multi-ring, multi-plane fixation for unstable fracture patterns. We conclude that a box hybrid or otherwise symmetric hybrid frame be used when hybrid external fixation of the tibia is considered.