Ankle Diastasis Screw - Chris Huber 4/4/2002

Mech of injury : Lauge-Hansen Pronation- External Rotation, or Weber C

"High ankle sprain" ( Hamilton )

Injury commonly missed.

Significant biomechanical joint abnormalities may occur with minimal XR disruption.

Normally, on wt-bearing 6-15% of load is borne by fibula, transmitted via interosseous membrane. Mortise widens 1mm on maximal ankle DF and fibula undergoes 2 degrees ER “ dynamic articulation.

Points to consider:

Need for a screw?

            No, not if medial and lateral malleoli are adequately fixed. Only likely to need if deltoid lig injured OR you are not fixing a medial #. Boden says even if deltoid lig torn, no need for diastasis screw if fibular # <4.5cm above joint (JBJS 71A 1989). Stress in ER or do "hook test" with I.I. but AFTER having fixed everything, incl medially.

What was that paper that everyone quotes?

            Ramsey and Hamilton, JBJS 58 A '76. They showed 1mm of lateral talar shift causes reduced contact area of tibiotalar joint by 42%. But criticised as it was in isolated cadaver tibia+talus with most of soft tissues removed

How far above joint?

            2cm better than 3.5cm above joint, see next sheet

How does it work?

            It's not a lag screw, it's a "position" screw rather than a "compression" screw

How many cortices?

            3 rather than 4 seems consensus. 4 cortices with a cortical screw will cause unwanted compression “ detrimental to ROM (Stiehl Foot and Ankle '89) Similarly, overtightening causes reduced DF

Position of ankle on screw insertion?

            Often said to be necessary to fully dorsiflex (max talar width in mortise) BUT   Tornetta et al (JBJS 83 A 2001) showed degree of ankle DF on insertion not important

What size screw?

            AO said 3.5mm diam, others since said 4.5mm. Biomechanical testing shows no advantage using 4.5mm screw (see next sheet)

When to remove?

            8 weeks seems better than 6 weeks. Several reports of loss of reduction when removed "early" at 6 weeks. NWB til removal to avoid screw breakage. Leaving screw in situ for longer periods gave no problems in several series, even if synostosis occurred, in the short term.

Bioresorbable implant?

            Several articles have shown no complications, and it avoids the need for 2 ^nd op to remove hardware. Maybe commoner in UK in future?