ASEPTIC LOOSENING

 Engh GA et al Classification and preoperative radiographic evaluation: Knee. CORR 1998

Engh GA et al Classification and preoperative radiographic evaluation: Knee. CORR 1998

Osteolysis is a process driven by polyethylene wear. Osteolysis is bone loss around the implant, which leads to implant loosening (there is no support) and periprosthetic fracture (thinner bone loses strength).

Osteolysis is less common in TKA as compared to THA, and while its still a common issue for TKA, particularly in the second and third decade of a TKA lifespan, it is not the most common complication (as in THA).  What is the difference between TKA and THA poly wear that leads to different rates of osteolysis?  TKA has different kinematics than THA, which leads to different poly wear patterns.  In THA, abrasive wear is the cause of poly debris.  In TKA, poly debris is generated by fatigue and delamination wear.  Delamination wear is repeated loading that causes subsurface cracks over time that propagate to form large debris particles (which are considerably less in number and less immunogenic).  

A TKA poly must be at least 6 mm thick to prevent fracture or significant delamination wear.  The only time you see abrasive wear in TKA is with backside wear (the backside of the poly rubs against the tibial tray that its locked into: however, the modern tibial trays are highly polished and therefore generate little debris).  While cross-linked poly is believed to have revolutionized THA wear and improved longevity, it is unlikely to have the same effect on TKA because cross-linked poly is most resistant to abrasive wear but there is no improvement to delamination.  In fact, there is concern that because cross-linked poly is more brittle (less ductile and therefore less resistant to fatigue crack propagation), it may be more susceptible to delamination wear.

Workup

Before making the diagnosis of aspetic loosening you must rule out septic loosening (ie infection) via inflammatory markers (ESR < 20, CRP < 10 mg/dL) and aspiration (only obtain if markers are elevated or high suspicion – negative result if synovial white cell count < 2,500 – 3000, differential PMN < 60%, alpha-defensin) [1] [2].

X-rays are obtained to look for bone loss, implant subsidence and alignment.  Oblique x-rays are helpful in identifying bone loss [3], advanced imaging like CT or MRI are not routinely obtained [4].

TKA bone defects are described by the AORI Classification system (Anderson Orthopedic Research Institute) [5].  There are three types.  Type 1 is a contained metaphyseal defects that doesn't impact the stability of a revision implant.  Type 2 is a metaphyseal defect in one femoral/tibial condyle (Type 2A) or both femoral/tibial condyles (Type 2B) requiring cement only (< 5 mm), cement + rebar (5 - 10 mm), or metal augments (>10 mm) to stabilize the implant. Type 3 is a significantly deficient metaphyseal region requiring cones or sleeves, or requiring the implant to bypass the metaphysis completely for stability, and often risks causing soft tissue instability (ie MCL deficiency). 

Treatment

Femoral bone loss.  Evaluate the medial and lateral condyle for 1) involvement of the epicondyles, which affects collateral ligament insertion and thus the level of implant constraint (PS vs. CCK vs. rotating hinge); and 2) central bone loss - the joint line should be ~25 mm from the lateral and ~30 mm from the medial epicondyle [6] and thus the need for augments vs. cones/sleeves. Cones attach directly to the component (like a super-stem), while sleeves are placed into the defect and the component is then cemented into the sleeve.

Tibial bone loss. Evaluate the tibial tubercle for risk of extensor mechanism injury and evaluate the depth of central bone loss (joint line is 15 mm from fibular head).  Defects < 10 mm can be treated with cement +/- pillar screws (4.5 mm cancellous) for rebar, if >10 mm, sleeves/cones are used to improve area of implant-bone interface [7] [8].  Sleeves and cones demonstrated >95% survival for AORI Type 2A, 2B and 3 defects.

Megaprosthesis for distal femoral replacement or proximal tibia replacement can be used for extensive bone loss, however, these implants risk higher rates of complications [9].