Candy box technique for the fixation of inferior pole patellar fractures: finite element analysis and biomechanical experiments

Abstract

Background: Maintaining effective reduction and firm fixation in inferior pole patellar fractures is a highly challenging task. There are various treatment methods available; although tension-band wiring combined with cerclage wiring (TBWC) is the mainstream approach, its effectiveness is limited. Herein, we propose and evaluate a new technique called candy box (CB), based on separate vertical wiring (SVW), for the treatment of inferior pole patellar fractures. Specifically, we provide biomechanical evidence for its clinical application.

Methods: Five fixation models were built: SVW combined with cerclage wiring (SVWC); TBWC; modified SVW with the middle (MSVW-A) or upper (MSVW-B) 1/3 of the steel wire reserved, and CB. A finite element analysis was performed to compare the displacement and stress under 100-N, 200-N, 300-N, 400-N and 500-N force loads. Three-dimensional printing technology was utilized to create fracture models, and the average displacement of each model group was compared under a 500-N force.

Results: The results of the finite element analysis indicate that CB technology exhibits significantly lower maximum displacement, bone stress, and wire stress compared to that with other technologies under different loads. Additionally, in biomechanical experiments, the average force displacement in the CB group was significantly smaller than that with other methods under a 500-N force (P < 0.05).

Conclusions: CB technology has the potential to overcome the limitations of current techniques due to its superior biomechanical characteristics. By incorporating early functional exercise and ensuring strong internal fixation, patient prognosis could be enhanced. However, further clinical trials are needed to fully evaluate the therapeutic effects of CB technology.

Keywords: Biomechanical experiments; Candy box technique; Finite element analysis; Fracture; Inferior Pole of the patella; Knee.

Fig. 1

Candy box technical schematic diagram. (A) Front view of candy box technology. (B) Oblique view of candy box technology. (C) Side view of candy box technology. (a) Three discontinuous vertical steel wires. (b) Upper 1/3 steel wires of the patellar tunnel. (c) Middle 1/3 steel wires of the patellar tunnel

Fig. 2

Diagram of different wiring techniques. (A) Separate vertical wiring combined with cerclage wiring (SVWC). (B) Tension-band wiring combined with cerclage wiring (TBWC). (C) Modified SVW with the middle 1/3 of the steel wire reserved (MSVW-A). (D) Modified SVW with the upper 1/3 of the steel wire reserved (MSVW-B). (E) Candy box technology (CB)

Fig. 3

Schematic diagram of patella loading. The inferior pole of the patella is bound and constrained, the upper pole is loaded, and the arrow indicates the stretching direction. The blue circles represent the support of the medial and lateral condyles of the femur to the patella

Fig. 4

Construction of physical models and biomechanical experiments. (A) Front view of fracture model. (B) Back view of fracture model. (C-D) Separate vertical wiring combined with cerclage wiring (SVWC). (E-F) Tension-band wiring combined with cerclage wiring (TBWC). (G-H) Modified SVW with the middle 1/3 of the steel wire reserved (MSVW-A). (I-J) Modified SVW with the upper 1/3 of the steel wire reserved (MSVW-B). (K-L) Candy box technology (CB)

Fig. 5

Histogram of displacement or stress at different forces. (A) The maximum displacement of different groups under different forces. (B) The maximum stress of patella under different forces. (C) The maximum stress of steel wire under different forces. Separate vertical wiring combined with cerclage wiring (SVWC). Tension-band wiring combined with cerclage wiring (TBWC). Modified SVW with the middle 1/3 of the steel wire reserved (MSVW-A). Modified SVW with the upper 1/3 of the steel wire reserved (MSVW-B). Candy box technology (CB)

Fig. 6

Displacement nephogram of different internal fixation groups under 500-N force. (A) Separate vertical wiring combined with cerclage wiring (SVWC). (B) Tension-band wiring combined with cerclage wiring (TBWC). (C) Modified SVW with the middle 1/3 of the steel wire reserved (MSVW-A). (D) Modified SVW with the upper 1/3 of the steel wire reserved (MSVW-B). (E) Candy box technology (CB)

Fig. 7

Stress nephogram of patella under 500-N force. (A) Separate vertical wiring combined with cerclage wiring (SVWC). (B) Tension-band wiring combined with cerclage wiring (TBWC). (C) Modified SVW with the middle 1/3 of the steel wire reserved (MSVW-A). (D) Modified SVW with the upper 1/3 of the steel wire reserved (MSVW-B). (E) Candy box Technology (CB)

Fig. 8

Stress nephogram of steel wire under 500-N force. (A) Separate vertical wiring combined with cerclage wiring (SVWC). (B) Tension-band wiring combined with cerclage wiring (TBWC). (C) Modified SVW with the middle 1/3 of the steel wire reserved (MSVW-A). (D) Modified SVW with the upper 1/3 of the steel wire reserved (MSVW-B). (E) Candy box technology (CB)

Fig. 9

Displacement diagram of different internal fixation groups under 500-N force. Separate vertical wiring combined with cerclage wiring (SVWC). Tension-band wiring combined with cerclage wiring (TBWC). Modified SVW with the middle 1/3 of the steel wire reserved (MSVW-A). Modified SVW with the upper 1/3 of the steel wire reserved (MSVW-B). Candy box technology (CB). *p < 0.05; ***p < 0.001