OTA 1998 Posters

Biomechanical Evaluation of the Fracture Stability between ORIF and Cement Augmentation with Proper Void Preparation in a Central Depressed Tibial Plateau Fracture

D.N. Yetkinler; T. McClellan; E.S. Reindel; D.R. Carter; R.D. Poser, Norian Corporation, Cupertino, CA; Santa Clara Valley Medical Center, San Jose, CA ; Stanford University, Biomechanical Engineering Division, Stanford, CA

Introduction: The purpose of this study was to compare the strength and stability of conventional ORIF with bone graft to fixation with a calcium phosphate cement in the treatment of a pure depression fracture of the lateral tibial plateau.

Materials and Methods: Twenty (20) pairs of fresh frozen human cadaver tibiae of varying bone quality were used. Bone quality of each specimen was assessed by dual energy X-ray absorptiometry (DEXA). The study was divided into two parts. The first part utilized thirteen (13) pairs of tibia. A control group receiving conventional fixation with bone graft and two cancellous screws were compared to an experimental group fixed with Norian SRS only. In the second part, seven (7) pairs of tibia were used to compare fixation with Norian SRS following rigorous void preparation versus Norian SRS following rigorous void preparation and two screws. Uniform pure depression fractures (AO type B2.2) were created in the lateral tibial plateau. Each tibia was loaded on an MTS with a slight valgus stance from 20 N to 250 N for 10,000 cycles to simulate immediate postoperative load transmission to the tibial plateau. Specimens were then loaded to failure to determine the strength of the reconstruction after dynamic loading. Intact medial tibial plateaus were tested to determine intact displacement and cyclic stiffness behavior during cyclic loading. These values were then normalized to the lateral plateau.

Results: In part 1, the articular fragment in the SRS specimens (2.4±1.1mm) were more stable than the conventionally treated control group (2.9±1.4 mm) after 10,000 loading cycles. The stiffness of Norian SRS (1368±223 N/mm) and Control constructions (1160±253 N/mm) were significantly less than the intact medial plateaus (2069±605 N/mm). While ultimate load to failure was similar for both groups, 63% more displacement of the articular fragment occurred in the control specimens (6.6±1.6 mm vs. 4.0±2.4 mm, p=0.05) by the time ultimate failure loads were attained. In part 2, the rigorous void preparation prior to SRS infiltration decreased the amount of settling after 10,000 loading cycles in both groups, SRS (1.1±0.5 mm) and SRS with screws (1.0±0.5 mm). The cyclic stiffness of the SRS with screws (1490±343 N/mm) was similar to the SRS group (1228±388 N/mm). Displacement during load to failure was similar in the SRS with screws (3.2±1.3 mm) and SRS group (2.8±2.0 mm). Comparing all four treatment groups in displacement during load to failure, control group was significantly weaker than all three SRS treated groups (p=0.05). In addition, intact bone stiffness (normalized) was significantly greater than the rest of the four experimental groups (p=0.05).

Discussion and Conclusion: These data demonstrate Norian SRS fixation of a pure central depression fracture of the lateral tibial plateau provides improved structural stability to that of conventional ORIF/bone graft fixation. Increased stability of the articular fragment was recorded during cyclic loading when compared to the conventional technique. When more rigorous void preparation was performed as in part 2, the settling results were further improved as compared to the part 1 result. This demonstrates attention to void preparation prior to SRS infiltration may improve the fracture construct stability. Other clinical advantages of SRS fixation include elimination of autograft related morbidity, reduction in both metallic fixation requirement, and elimination of hardware removal procedures.