Methodology and Hypothesis Testing was performed on full-scale anatomic models consisting of fourth-generation composite tibiae and tali. This

was an experimental design with each of the specimens prepped for ankle fusion and fixated with the following

constructs: 1) Tripod construct 2) Anterior plate 3) Anterior plate with compression screw 4) Lateral plate 5) Lateral

plate with compression screw (See Figure1 ). Each construct had 5 specimens.

Statement of Purpose

Ankle arthrodesis is a standard treatment for end-stage arthritis and deformity of the ankle joint. It restores function,

provides a stable, well-aligned, pain-free joint(1,2).

Various fixation techniques have been employed, including cross screws(1), external fixation, intramedullary nails and

plates( 2,3,4).

The most common form of fixation is with two or more internal fixation screws(4,5).

Regardless of the form of fixation, it is well documented that a successful osseous fusion relies on bony apposition,

compression, and rigid immobilization(1,2,5). Primary stiffness and contact area of the fusion are good predictors of

solid fusion(6).

Successful union has been reported between 94% to 100% for plate fixation(7,8,9).

Recent developments in plating technology have presented new options for fixation; however studies comparing the

various forms of locking plate fixation are few. Furthermore, no studies are available that compare the use of locking

plates with and without interfragmentary screws.

Biomechanical Comparison of Tibiotalar Arthrodesis Fixation Techniques Scott Berg, DPM/PGY21; Craig Clifford DPM2; Kevin McCann, DPM3; Byron Hutchinson DPM1

1Franciscan Foot & Ankle Institute, Federal Way, WA 2Franciscan Orthopedic Associates, Federal Way, WA

3Saint Cloud Orthopedics, Sartell, MN

References 1. Kakarla G, Rajan DT. Comparative study of ankle arthrodesis using cross screw fixation versus anterior contoured plate plus cross screw fixation. Acta Orthop Belg. 72(6):716-721, 2006.

2. Mueckley TM, Eichorn S, Oldenburg G, Speitling A, DiCicco J, Hofmann GO, Buhren V. Biomechanical evaluation of primary stiffness of tibiotalar arthrodesis with intramedullary compression nail and four other fixation devices.

Foot Ankle Int. 27(10): 814-820,2006.

3. Fragomen AT, Myers KN, Davis N. A biomechanical comparison of micromotion after ankle fusion using 2 fixation techniques: Intramedullary arthrodesis nail or Ilizarov external fixator. Foot Ankle Int. 29(3): 334-341, 2008.

4. Ogut T, Glisson RR, Chuckpaiwong B, Le I, Easley ME. External ring fixation versus screw fixation for ankle arthrodesis: A biomechanical comparison. Foot Ankle Int. 30(4):353-360, 2009.

5. Dohm MP, Benjamin JB, Harrison J, Szivek JA. A biomechanical evaluation of three forms of internal fixation used in ankle arthrodesis. Foot Ankle Int. 15: 297-3001994.

6. Hintermann B, Valderrobano V, Nigg B. Influence of screw type on obtaining contact area and contact force in a cadaveric subtalar arthrodesis model. Foot Ankle Int. 23:986-991, 2002.

7. Braly, WG, Baker JK, Tullos HS. Arthrodesis of the ankle with lateral plating. Foot Ankle 15:649-653, 1994.

8. Scranton PE. Use of internal compression in arthrodesis of the ankle. J. Bone Joint Surg. 67A: 550-555, 1985.

9. Sowa, DT, Krackow KA. Ankle Fusion: A new technique of internal fixation using a compression blade plate. Foot Ankle. 9:232-240, 1989.

10. Friedman RL, Glisson, RR, Nunley J. A biomechanical comparative analysis of two techniques for tibiotalar arthrodesis. Foot Ankle Int. 15:301-305, 1994.

11. Schuberth, JM, Ruch JA, Hansen ST. The tripod fixation technique for ankle arthrodesis. Journal of Foot and Ankle Surgery. 48(1):93-96, 2009.

12. Jeng CL, Baumbach SF, Kalesan B. Comparison of initial compression of the medial, lateral, and posterior screws in ankle fusion construct. Foot Ankle Int. 32(1):71-6, 2011.

13. Nasson S, Shuff M, Palmer D, Owen J, Wayne J, Carr J, Adelaar R, May D. Biomechanical comparison of ankle arthrodesis techniques: Crossed screws vs. blade plate. Foot Ankle Int. 22:575-580, 2001.

Analysis and Discussion Our data shows that the lateral plate with compression screw resisted inversion and eversion the best, the anterior plate with

compression screw resisted dorsiflexion and plantarflexion the best and had the greatest bending stiffness overall.

In conclusion, anterior plate with compression screw and lateral plate with compression screw were shown to have the most

overall bending stiffness. Plate fixation was shown to have the greatest resistance to bending when placed on the tension

side, and bending resistance was significantly improved with the use of a compression screw.

Procedures

All 25 specimens were tested with a standardized protocol using an Instron 4505 Universal Testing System(Instron,

Norwood MA). Tibia were affixed to the load cell, and potted tali were bolted to a rigid steel cantilever beam. This

beam was allowed to freely pivot on a fulcrum mounted to the crosshead of the testing machine 12cm from the

center of the ankle joint(Figure 2). The crosshead was raised 5.3mm at a rate of 1mm/sec to place a bending

moment on the arthrodesis site of 3 degrees. The apparatus was rotated to place the fulcrum anterior, medial,

posterior, and lateral to the arthrodesis site for respective bending in modes of dorsiflexion, inversion,

plantarflexion, and eversion (Figure 3) . Two trials of each bending mode were performed for each specimen, with

the second trial being recorded. Force-displacement data were captured and recorded every 10ms and curves

plotted using Bluehill software (Instron, Norwood, MA).

The slope of each curve was used as representation of bending stiffness(N/mm). Groups were compared using

one-way ANOVA with Tukey post-hoc tests.

Results

Overall testing showed that a plate with a compression screw had significantly greater stiffness than a plate alone, or three

compression screws (Graph 1). There was no significant difference between the anterior plate with compression screw or the

lateral plate with compression screw (α < 0.05). There was no significant difference between the anterior plate, lateral plate, or

three compression screws (α < 0.05).

The greatest dorsiflexion bending stiffness observed was the anterior plate with compression screw. This is 71% greater than

the anterior plate alone (Table 1). The greatest plantarflexion bending stiffness observed was anterior plate. This was not

significantly different from the anterior plate with compression screw. The greatest inversion bending stiffness observed was

lateral plate with compression screw. This was not significantly different from the lateral plate alone. The greatest eversion

bending stiffness observed was the lateral plate with compression screw. This was not significantly different from the anterior

plate with compression screw, but was 74% greater than the lateral plate alone (Table 1).

Fixation Method

Dorsiflexion

(N/mm)

Plantarflexion

(N/mm)

Inversion

(N/mm)

Eversion

(N/mm)

Overall

(N/mm)

Std Dev(σ) Std Dev(σ) Std Dev(σ) Std Dev(σ) Std Dev(σ)

Anterior Plate +

Compression Screw

60.76 65.53* 30.92* 47.75* 51.24*

σ = 12.63 σ = 12.52 σ = 2.60 σ = 7.07 σ = 16.37

Lateral Plate +

Compression Screw

39.86 40.99 65.11† 49.29* 48.82*

σ = 3.62 σ = 8.10 σ = 2.32 σ = 8.22 σ = 11.79

Anterior Plate 17.55 66.07* 31.12* 36.19 37.73†

σ = 1.01 σ = 12.83 σ = 5.78 σ = 3.52 σ = 19.37

Lateral Plate 29.51* 30.70† 57.58† 12.82 32.65†

σ = 7.12 σ = 7.74 σ = 4.49 σ = 3.02 σ = 17.32

Three Compression

Screws

30.09* 26.27† 29.02 40.58* 31.49†

σ = 4.62 σ = 8.11 σ = 3.72 σ = 10.19 σ = 8.61

* no significant difference

† no significant difference

Figure 3. Potted tail with fixated tibia with rigid steel cantilever

beam

Table 1. Results Graph 1. Overall stiffness (N/mm)

Figure 1. AP and Lateral of each Construct

Figure 2. Load cell with

cross beam

Literature Review

Friedman et al(10) evaluated the biomechanical comparison between 2 crossed screws and 2 parallel screws for

ankle arthrodesis on cadaveric specimens. They found that the two parallel screws were more rigid in resisting

plantarflexion and inversion and that the two crossed screws were more rigid in resisting eversion and dorsiflexion

and internal and external rotation. The use of cadaveric specimens were matched pairs to optimize the validity of

compression but the specimens were not subjected to any bone density testing.

Schuberth et al(11) described the technique for fixation using the tripod configuration.

Jeng, et al(12) evaluated the effect of order of screw insertion within a tripod construct. They found no significant

difference in contact area or compression between for initial insertion of medial, lateral, or posterior screw.

Nasson et al(13) evaluated the biomechanical comparison between 2 crossed screws and a blade plate on older

generation saw bone models. Their study found that crossed screws had greater stiffness during dorsiflexion and

valgus loading. This was significant because neutral to slight valgus fusion position is desired and dorsiflexion is

the main stressor during gait.