Session VIII - Basic Science

Pauwels' III Femoral Neck Fractures: Biomechanical Study of 4 Fixation Techniques

Arash Aminian, MD (n); Fan Gao, PhD (n); Wasyl W. Fedoriw, BS (n); Bradley R. Merk, MD (n); Li-Qun Zhang, PhD (n);

Northwestern University, Chicago, Illinois, USA

Purpose: Pauwels' type 3 femoral neck fractures are problematic to treat, with complication rates as high as 30% to 60% reported in the literature. The purpose of this study was to compare four constructs 7.3-mm cannulated screws inserted in an inverted triangle, 135° dynamic hip screw construct (DHS), 95° dynamic condylar screw construct (DCS), and the fixed-angle proximal locking femoral plateto determine the biomechanically most stable fixation in a Pauwels' III femoral neck fracture.

Methods: Eight fresh cadaveric specimens were allocated to the 4 groups after a preliminary power analysis. All specimens were DEXA scanned. The average bone densities of each group fell within parameters of normal bone density based on peak range of young adults. A 90^o angled vertical osteotomy was created simulating a Pauwels' III femoral neck fracture. The osteotomies were internally fixed under fluoroscopic guidance. The specimens were mounted in 25° of adduction to stimulate one leg stance. Using a motor coupled sensor, a vertical force was applied along the mechanical axis of the bones in three modes: progressive loading up to 1400 N, cyclical loading 1400 N at 3 Hz for 10,000 cycles, and loading to failure. Failure was defined as fracture displacement more than 5 mm, hardware fracture, screws backing out more than 5 mm, or head rotation more than 5°. Displacement was measured using an OPTOTRAK system and markers on the bones. The stiffness, failure loads, and failure energy were calculated from the load versus displacement plots. Statistical analysis was performed using the ANOVA test and post-hoc test with significance level set at P <0.05.

Results: The results of the three phases of the experiment are summarized in the following table.

Biomechanical Strength of Fixation Techniques

Groups	Instrumentation	Stiffness N/mm	Failure Strength N	Failure Energy J	Failure During Cycling # of Bones
1	7.3-mm cannulated screws	165.58±50.25	861.83±365.73	134.16±79.29	N/A*
2	DHS	245.43±50.69	1193.74±213.38	196.63±75.74	3: (1822), (1087), (7450)#
3	DCS	319.93±46.41	2156.38±436.49	247.12±137.50	0
4	Locking Plate	618.34±164.10	2430.83±615.58	406.15±199.30	0

*All of the bones in group 1 failed during initial loading

#5 of 8 bones in group 2 failed during initial loading

The 4 groups were statistically different. Based on the data, the strongest construct was the locking plate and the weakest construct was the 7.3-mm cannulated screw configuration.

Conclusion/Significance: Biomechanically the strongest construct for stabilizing a Pauwels' III femoral neck fracture is the proximal femoral locking plate, followed in descending order by the dynamic condylar screw, the dynamic hip screw, and the 3 cannulated screw configuration. The DCS and locking plate constructs tolerated cyclical loading at 1400 N for the 10,000 cycles, which corresponds to 2 months of healing time. This is significant because it allows time for the fracture to heal with full weight bearing at physiologic loads. However, the average displacement after cyclically loading was significantly less in the locking plate group compared to the displacement in the DCS group, with displacements being 1.71 ± 0.80 mm and 3.59 ± 1.18 mm respectively (P <0.002). Clinically the locking plate may prove to be a superior construct when treating Pauwels' III femoral neck fractures, decreasing the high rates of complications seen with these injuries and improving patient outcomes.