OTA 1997 Posters - Foot & Ankle Fractures

*Superior Compressive Strength of Calcaneus Fracture Construct Augmented with Remodelable Cancellous Bone Cement

David B. Thordarson, MD, Thomas P. Hedman, PhD, Duran N. Yetkinler, MD, PhD, Enass Eskander, MD, Bud Lawrence, BS, Robert D. Poser, DVM

LAC and USC Medical Center, Los Angeles, California, USA

Purpose: The purpose of the study was to evaluate the compressive strength of a calcaneus fracture reconstruction augmented with a new, remodelable cancellous bone cement compared to standard fixation methods. In addition, an intra-articular calcaneus fracture model with bone impaction was developed.

Methods: Twenty-six paired fresh-frozen cadaveric feet were disarticulated from the leg, soft tissues removed from the medial, lateral, and posterior aspects of the calcaneus, and the talus potted in polyurethane. Eighteen specimens had good bone quality and eight were osteoporotic. This determination was made empirically in all specimens (resistance to creation of stress risers, screw insertion torque, etc.). An extensometer was mounted on the medial aspect of the calcancus between two Schanz pins placed in the calcaneus along its superior and inferior aspects. Specimen baseline stiffness was determined in an MTS testing machine by loading for ten cycles to 100 N and ten cycles to 350 N. Baseline x-rays of the specimen were obtained. Stress risers were placed along the medial, lateral, and posterior aspects of the calcancus with a narrow osteotome to simulate a displaced Sander's II B two-part intra-articular fracture (JOT 73-Cl.2). The specimen was loaded to failure in an Instron machine in displacement control after pre-loading to 100 N and then causing a displacement of 20 mm at a rate of 40 mm/second. One specimen of each pair was randomly chosen and underwent standard operative fixation using two subchondral, 3.5 mm cortical screws and by applying a contoured Y-plate (Synthes; Paoli, PA) with appropriate length 3.5 mm screws after reducing the body of the calcaneus. The void in the body of the calcaneus was filled with cancellous bone graft. The matched specimen of each pair underwent surgical fixation employing the same hardware, but the void in the calcancus was filled with Norian SRS (Skeletal Repair System) remodelable cancellous bone cement injected through a 12-gauge needle. The specimen was not disturbed for ten minutes during initial cement curing. Radiographs were performed after fracture creation and fracture reconstruction with a subsequent CT scan. All specimens were cured in a 37°C saline bath for 24 hours. All specimens were cyclically tested for 10 cycles to 100 N and 10 cycles to 350 N at 0.2 Hz followed by 1000 cycles at 350 N at 0.2 Hz or to failure (7 mm or more displacement). The first cycle deformation (mm) and deformation per cycle (mm/cycle) were calculated. Statistical significance was evaluated with a Wilcoxon rank sum test and p < 0.05 considered significant.

Results: The results of the deformation per cycle in all specimens, the good and poor bone quality specimens are presented in Table I.

TABLE I

Deformation/Cycle (mm/cycle)
	All (n = 26)	Good bone (n = 18)	Poor bone (n = 8)
SRS	0.00195 ± 0.00351	0.000727 ± 0.00114	0.00476 ± 0.00591
Control	1.0130 ± 2.0211	0.33086 ± 0.7701	3.675 ± 3.850
p value	0.0047	0.028	0.068

The results of the first cycle deformation for all the specimens are presented in Table II.

TABLE II

1st Cycle Deformation (mm)
SRS	0.478 ± 0.489
Control	3.090 ± 2.797
p value	0.011

Further analysis of load to failure data in the good bone specimens demonstrate significant (p < 0.014) improvement in fracture stability in the SRS specimens. Six of nine control specimens displaced 2.5 mm or greater (range 2.5 - 7.0 mm) prior to 1000 cycles, with three of these specimens displacing 7 mm prior to completion of loading. However, only one of the nine SRS specimens displaced significantly (3.2 mm), and all SRS specimens reached 1000 cycles. A reproducible Sander's IIB intraarticular displaced fracture (JOT73-Cl.2) was created. An anatomic reconstruction was achieved for all specimens - Bohler's angle pre-fracture 37° (avg.) vs. following reconstruction 36° (avg.). The CT scans of specimens augmented with SRS bone cement revealed good filling of the bony void. Radiographs after failure testing did not reveal any visible change in the bone cement mass in any specimen. In the SRS specimens, failure occurred by impaction of the cancellous bone around the cement mass. In the control specimens, failure occurred with impaction of the bone primarily in the defect which had been grafted.

Discussion: Many surgeons now achieve superior results with operative fixation of calcaneus fractures compared to nonoperative treatment. Operative treatment of calcaneus fractures frequently includes autogenous bone grafting to fill the bony void with its increased morbidity and hospital stay. Weight bearing is generally delayed for 10 to 12 weeks following surgery due to the lack of the initial surgical construct stability.

Norian SRS® (skeletal repair system) is an injectable in-situ setting calcium phosphate cancellous bone cement. It initially hardens in 10 minutes with full curing over 24 hours and provides excellent initial compressive strength of 55 MPa. It has been shown to be biocompatible and susceptible to natural osteoclastic/osteoblastic remodeling. Our results demonstrated a dramatic increase in the initial compressive strength of a calcaneus fracture construct augmented with SRS bone cement compared to standard fixation by evaluating the deformation per cycle and the first cycle deformation. Comparison in good and poor bone quality specimens revealed similarly impressive results. CT scans of the specimens revealed good filling of the bony void after the SRS bone cement was injected through a 12-gauge needle. No specimen revealed any evidence of damage to the bone cement mass after failure testing.

Conclusion: A dramatic increase in the compressive strength of the calcaneus fracture construct was noted in specimens where standard operative fixation methods were augmented with SRS cancellous bone cement. Similarly impressive results were noted in both good and poor bone quality specimens. Clinical use of this new, cancellous bone cement could markedly increase the initial fracture construct strength, which could thus lead to more rapid rehabilitation following this difficult fracture with a shorter disability period. The SRS bone cement would additionally obviate the need for autogenous bone grafting, thus shortening the hospital stay with significant monetary savings and decreased morbidity.