NOVEL SCREW DESIGNS PROVIDE BETTER FIXATION STABILITY OF SCAPHOID FRACTURES: AN FE STUDY

Peter Varga (1,2), Philip Schefzig (3), Ewald Unger (4), Winfried Mayr (4), Philippe K. Zysset (1), Jochen Erhart (3)

1. ILSB, TU Vienna, Austria; 2. JWI&BCRT, Charité - UM Berlin, Germany;

3. DTS, MU Vienna, Austria; 4. CBEPh, MU Vienna, Austria

Introduction

The scaphoid is the most often fractured carpal

bone. Scaphoid fractures compromise stability and

function of the wrist. Non-union is a serious clinical

problem [Slade&Dodds, 2006]. Single screw

osteosynthesis is performed to complement or to

replace the conservative cast immobilization in

order to foster healing. Mechanical stability of this

technique may be insufficient for optimal healing.

In particular, rotation around the screw axis may

not be sufficiently restricted and result in unwanted

shear-type inter-fragmentary motion (IFM). Special

screw designs including extra elements attached to

the main screw body were hypothesised to provide

enhanced rotational stability of the fragments. This

assumption was tested by means of CT image-

based finite element (FE) modelling.

Methods

A cadaveric forearm was scanned in eight different

wrist positions with HR-pQCT while loading

specific muscles with weights and the carpal bone

positions were extracted by image processing.

Following dissection, the carpal bones were μCT

scanned and accurate local articular cartilage

thicknesses were measured. Images of the two

modalities were combined, which resulted in

overlaps of cartilage layers. Cartilage indentation

was resolved and converted into contact forces

acting on the scaphoid by means of an FE model of

the wrist. Ligament forces were computed to

provide equilibrium. All forces were transferred to

a detailed, μCT image-based FE model of the

scaphoid. A virtual osteotomy was introduced and

screw fixations were included. IFMs were

computed for one conventional (A) and two novel

(B and C) screw designs in two wrist positions,

loaded neutral (LN) and total extension (TE).

Results

Mean±SD cartilage thickness of the scaphoid were

0.71±0.18 mm. Cartilage contact stresses acting on

the scaphoid are shown in Figure 1, left. In-plane

IFMs were dominantly rotational around the screw

axis (Fig 1, right) and were approximately 50%

smaller for the new screw designs (B and C)

compared to the conventional screw A (Table 1).

Figure 1: Left: scaphoid cartilage contact stresses

(in MPa) in neutral wrist position. Right: IFM at

the osteotomy plane: out-of-plane (colour plot) and

in-plane (arrows, 100x magn.) components.

Wrist position Screw A Screw B Screw C

Loaded neutral 0.231 0.077 0.078

Total extension 0.101 0.042 0.065

Table 1: Rotational IFM (in degrees) in LN and TE.

Discussion

Main limitations of the study are that only a single

specimen was used and that no experimental

validation was performed. Results may not

represent a general in vivo situation. Distribution

and magnitude of cartilage contact stresses were in

line with previous studies, but may be more

realistic than earlier numerical results due to the

accurately identified bone positions and cartilage

thicknesses. IFMs found here were in line with

previous results [Ezqerro, 2007]. Even if not the

exact values, the orders of magnitude and the trends

found here should be valid. The novel screws

efficiently restricted in-plane IFMs and have a great

potential to allow for faster and better healing. This

should however be verified clinically.

Acknowledgements

Austrian Science Fund (FWF), grant L613-N14.

References

Slade and Dodds, Clin Orthop Relat Res, 445:108-

19, 2006.

Ezquerro et al. Med Eng Phys 29:652-60, 2007.

S318 Presentation 1189 − Topic 25. Implant biomechanics

Journal of Biomechanics 45(S1) ESB2012: 18th Congress of the European Society of Biomechanics