MCQs – Knee Recon 3

MCQs-Recon Knee
Overdrilling the hole before using IM alignment guides reduces fat embolism and hypoxia.
Insertion of the IM rods, particularly in the femur, is thought to be associated with fat embolism. When rods are used, the canal contents should be removed with suction before insertion. It is also important to vent the canal through either an enlarged hole at the surface of the knee or some other technique to decompress the canal.

Metal backed patellas are bad all around – increase poly stress and metallic synovitis. They have increased ply wear, increased fracture, peg-plate dissociation, poly-metal dissociation, metal plate fracture, metal wear, and synovitis.

Primary resurfacing of patella seems to give better long term results than leaving it unresurfaced.

Single peg patellar components seem to be associated with a higher rate of patellar bone fracture than the triple peg components.

Patellar fracture:
– if minimally displaced and ext. mechanism intact, then treat with immobilization
– if displaced and component unstable – fix and reconstruct patella
– if comminuted – excise bone, repair extensor mechanism??

A number of things seem to affect long-term prognosis in TKA – age over 70 is good! Patellar replacement is good. Alignment is key!

Titanium makes for a poor bearing surface, but is good for ingrowth.

Pore size is critical in terms of bony ingrowth – 50-100 results in fibrous ingrowth. Optimal pore size is between 100-250 microns.

Porous coating and bony ingrowth is amenable for the femur only.

In a valgus knee – plan on releasing tensor fascia lata, LCL, popliteus, and possibly posterolateral complex

A TKA that is loose in flexion, and will not come into full extension – need to insert a bigger polyethylene tray, and resect more of the distal femur.

A TKA that is tight in extension, good in flexion – resect more off the distal femur; can also try to release abit of the posterior capsule

A TKA that is tight in flexion and in extension – cut more off tibia (lowers the joint line abit though)

Patellar problems in TKA include instability, erosion, fracture, loosening, and pain.

The main reason to keep the PCL is to improve stair climbing by providing adequate rollback.

If you cut the PCL – no big deal – convert it to a posterior stabilized design.

Metal backed patellas are generally BAD – have to resect more patella to accommodate the thickness of the metal, or you would have a thin poly. High rates of failure.

Unicompartmental knee replacement tends to leave more bone stock than TKA. Best for the elderly, thin (non-obese) with predominantly unilateral gonarthrosis, intact cruciates, and frontal plane malalignment less than 15 degrees, with at least 90 degrees of ROM.

Early wound slough requires immediate irrigation, debridement, and coverage with medial gastrocs flap.

For the tibial component, metal backing is good to decrease stress on bone cement; cortical support is important.

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