Fragment size of lateral Hoffa fractures determines screw fixation trajectory: a human cadaveric cohort study

Abstract

Background and purpose: Recommendations regarding fragment-size-dependent screw fixation trajectory for coronal plane fractures of the posterior femoral condyles (Hoffa fractures) are lacking. The aim of this study was to compare the biomechanical properties of anteroposterior (AP) and crossed posteroanterior (PA) screw fixations across differently sized Hoffa fractures on human cadaveric femora.

Patients and methods: 4 different sizes of lateral Hoffa fractures (n = 12 x 4) were created in 48 distal human femora according to the Letenneur classification: (i) type I, (ii) type IIa, (ii) type IIb, and (iv) type IIc. Based on bone mineral density (BMD), specimens were assigned to the 4 fracture clusters and each cluster was further assigned to fixation with either AP (n = 6) or crossed PA screws (n = 6) to ensure homogeneity of BMD values and comparability between the different test conditions. All specimens were biomechanically tested under progressively increasing cyclic loading until failure, capturing the interfragmentary movements via motion tracking.

Results: For Letenneur type I fractures, kilocycles to failure (mean difference [∆] 2.1, 95% confidence interval [CI] -1.3 to 5.5), failure load (∆ 105 N, CI -83 to 293), axial displacement (∆ 0.3 mm, CI -0.8 to 1.3), and fragment rotation (∆ 0.5°, CI -3.2 to 2.1) over 5.0 kilocycles did not differ significantly between the 2 screw trajectories. For each separate subtype of Letenneur type II fractures, fixation with crossed PA screws resulted in significantly higher kilocycles to failure (∆ 6.7, CI 3.3-10.1 to ∆ 8.9, CI 5.5-12.3) and failure load (∆ 275 N, CI 87-463 to ∆ 438, CI 250-626), as well as, less axial displacement from 3.0 kilocycles onwards (∆ 0.4°, CI 0.03-0.7 to ∆ 0.5°, CI 0.01-0.9) compared with AP screw fixation.

Conclusion: Irrespective of the size of Letenneur type II fractures, crossed PA screw fixation provided greater biomechanical stability than AP-configured screws, whereas both screw fixation techniques demonstrated comparable biomechanical competence for Letenneur type I fractures. Fragment-size-dependent treatment strategies might be helpful to determine not only the screw configuration but also the surgical approach.

Figure 1

Schematic illustration of the Letenneur classification. Type I shows a fracture line through the lateral femoral condyle that is in line with the posterior cortex and involves 100% of the posterior femoral condyle. Type II are osteochondral fractures of the lateral femoral condyle with its subclassifications moving more posteriorly (IIa, IIb, and IIc—involving 75%, 50%, and 25% of the posterior femoral condyle). Type III depicts an oblique fracture line intersecting the articular surface more anteriorly.

Figure 2

Mediolateral radiographs of exemplified specimens with anteroposterior (left) or crossed posteroanterior (right) screw fixation visualizing the different size-dependent modeling of lateral Hoffa fractures, according to the Letenneur classification. Letenneur type I, IIa, IIb, and IIc—involving 100%, 75%, 50%, and 25% of the posterior lateral femoral condyle, respectively.

Figure 3

Mediolateral (a, c) and anteroposterior (b, d) radiographs of exemplified specimens with (a, b) anteroposterior and (c, d) crossed posteroanterior screw fixations.

Figure 4

Setup with a specimen mounted for biomechanical testing.

Figure 5

Axial displacement of the different types of Hoffa fractures following anteroposterior (AP) or crossed posteroanterior (PA) screw fixation, presented as mean value and standard deviation over the course of cyclic testing, together with the corresponding P value from the statistical comparisons between groups.

Figure 6

Fracture gap twisting of the different types of Hoffa fractures following anteroposterior (AP) or crossed posteroanterior (PA) screw fixation, presented as mean value and standard deviation over the course of cyclic testing, together with the corresponding P value from the statistical comparisons between groups.

Figure 7

Fracture gap opening of the different types of Hoffa fractures following anteroposterior (AP) or crossed posteroanterior (PA) screw fixation, presented as mean value and standard deviation over the course of cyclic testing, together with the corresponding P value from the statistical comparisons between groups.

Figure 8

Kilocycles to failure and corresponding failure load (kN) of the different types of Hoffa fractures following anteroposterior (AP) or crossed posteroanterior (PA) screw fixation, presented as mean value and standard deviation, together with the corresponding P value from the statistical comparisons between groups.