Ramsey and Hamilton measured how the contact area between tibia and talus changes as the talus is displaced laterally within the ankle mortise.
Cadaveric study of 23 ankles from amputated limbs, contact mapped with carbon black at neutral and at 1, 2, 4 and 6 mm of lateral talar shift.
The authors postulated that talar displacement alters tibiotalar contact area and that this may contribute to poor results.
Methods
Model
23 lower limbs amputated for peripheral vascular disease
Specimens with infection or gangrene of ankle cartilage excluded
Fibula removed
Ligaments, capsule and all other soft tissue excised
Tibia transected 5 cm above the joint
Constructs
Talus in neutral position
Talus displaced laterally 1, 2, 4 and 6 mm
Outcome measures
Tibiotalar contact area by carbon black transference at each displacement
Location of contact on the talus
Key Findings
Contact area fell by a mean of 42% with just 1 mm of lateral talar shift, the largest loss of any step.
Lateral shift reversed the contact pattern, so contact became broad medially and narrow laterally.
Neutral contact area varied considerably between specimens, which the authors attributed to normal biological variation.
Board pearl
Once the talus shifted laterally, contact was confined to the medial and lateral talar prominences, with no measurable contact in the mid‑portion of the talus.
The first millimeter of shift cuts contact by 42%Ramsey, J Bone Joint Surg Am 1976
Mean tibiotalar contact area (cm²)
The first millimeter of shift cuts contact by 42%
Contact area 0mm
4.4cm²
Contact area 1mm
2.5cm²
Contact area 2mm
1.89cm²
Contact area 4mm
1.53cm²
Contact area 6mm
1.37cm²
Authors' conclusion
Since the stress per unit area increases as the total contact area decreases, a decrease in contact area may be a factor contributing to a poor result after ankle fracture or dislocation when talar displacement is one millimeter or more.
Ramsey and Hamilton measured how the contact area between tibia and talus changes as the talus is displaced laterally within the ankle mortise.
Cadaveric study of 23 ankles from amputated limbs, contact mapped with carbon black at neutral and at 1, 2, 4 and 6 mm of lateral talar shift.
The authors postulated that talar displacement alters tibiotalar contact area and that this may contribute to poor results.
Methods
Model
23 lower limbs amputated for peripheral vascular disease
Specimens with infection or gangrene of ankle cartilage excluded
Fibula removed
Ligaments, capsule and all other soft tissue excised
Tibia transected 5 cm above the joint
Constructs
Talus in neutral position
Talus displaced laterally 1, 2, 4 and 6 mm
Outcome measures
Tibiotalar contact area by carbon black transference at each displacement
Location of contact on the talus
Key Findings
Contact area fell by a mean of 42% with just 1 mm of lateral talar shift, the largest loss of any step.
Lateral shift reversed the contact pattern, so contact became broad medially and narrow laterally.
Neutral contact area varied considerably between specimens, which the authors attributed to normal biological variation.
Board pearl
Once the talus shifted laterally, contact was confined to the medial and lateral talar prominences, with no measurable contact in the mid‑portion of the talus.
The first millimeter of shift cuts contact by 42%Ramsey, J Bone Joint Surg Am 1976
Mean tibiotalar contact area (cm²)
The first millimeter of shift cuts contact by 42%
Contact area 0mm
4.4cm²
Contact area 1mm
2.5cm²
Contact area 2mm
1.89cm²
Contact area 4mm
1.53cm²
Contact area 6mm
1.37cm²
Authors' conclusion
Since the stress per unit area increases as the total contact area decreases, a decrease in contact area may be a factor contributing to a poor result after ankle fracture or dislocation when talar displacement is one millimeter or more.