As the screw is tightened, the thread advances in the main tibial fragment and this pulls the head of the screw, and therefore the malleolar fragment, towards the main body of the tibia.
The smooth shaft of the screw prevents any significant hold between itself and the surrounding bone.
The length of the screw shaft must be chosen so that the threaded part of the screw lies fully within the opposite bone fragment, but does not penetrate beyond the denser epiphyseal cancellous bone.
To prevent the screw head from sinking into the thin malleolar cortex, the use of a washer is recommended.
It is important to visualize and inspect the joint, searching for any bone and cartilage fragments that may have been detached from the medial malleolus or from the talus.
Reduce the fracture anatomically with the use of small pointed reduction forceps, taking care with the soft tissues.
To monitor the perfect articular reduction, a limited antero-medial arthrotomy will permit visual inspection of the joint surface.
Do not strip the periosteum more than necessary.
Insert a 1.6 mm K-wire as perpendicularly as possible to the fracture plane.
As decided in preoperative planning, this K-wire should occupy the planned position of the posterior lag screw.
Check the reduction visually (especially anteriorly) and under image intensification.
Make a stab incision through the deltoid ligament. With the protection of the drill sleeve, drill a 2.5 mm hole as perpendicularly as possible to the fracture plane, and parallel and anterior to the K-wire. Do not drill to the lateral cortex.
Care must be taken to avoid penetration of the ankle joint.
Keeping the drill in place, check its position and the reduction under image intensification.
Measure the drill depth and tap the malleolar fragment only with the 4.0 mm cancellous bone tap, using the protection sleeve. The length of the screw chosen is based on the principle that all thread must lie proximal to the fracture plane. Be careful not to choose a screw that is too long, as its thread will come to lie in the fattier cancellous bone of the metaphysis, rather than in the denser bone of the former epiphysis.
The chosen 4.0 mm cancellous bone screw should come to rest with its threads completely beyond the fracture line.
The use of a washer is recommended, especially in osteoporotic bone.
Insert the screw without excessive tightening.
Remove the K-wire. Make a stab incision through the deltoid ligament and enlarge the K-wire track with a 2.5 mm drill and protection sleeve.
After measuring the length and tapping the malleolar fragment, insert the second chosen 4.0 mm cancellous bone screw.
The fracture will be suitably fixed with the two cancellous bone screws and the joint surface reconstructed.