Abstract
Constrained implants with intra-medullary fixation are expedient for complex TKA. Constraint is associated with loosening, but can correction of deformity mitigate risk of loosening?
Primary TKA's with a non-linked constrained prosthesis from 2010-2018 were identified. Indications were ligamentous instability or intra-medullary fixation to bypass stress risers. All included fully cemented 30mm stem extensions on tibia and femur. If soft tissue stability was achieved, a posterior stabilized (PS) tibial insert was selected.
Pre and post TKA full length radiographs showed.
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i.
hip-knee-ankle angles (HKAA)
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ii.
Kennedy Zone (KZ) where hip to ankle vector crosses knee joint.
77 TKA's in 68 patients, average age 69.3 years (41-89.5) with OA (65%) post-trauma (24.5%) and inflammatory arthropathy (10.5%).
Pre-op radiographs (62 knees) showed varus in 37.0% (HKAA: 4o-29o), valgus in 59.6% (HKAA range 8o-41o) and 2 knees in neutral.
13 cases deceased within 2 years were excluded. Six with 2 year follow up pending have not been revised. Mean follow-up is 6.1 yrs (2.4-11.9yrs).
Long post-op radiographs showed 34 (57.6%) in central KZ (HKKA 180o +/- 2o).
Thirteen (22.0%) were in mechanical varus (HKAA 3o-15o) and 12 (20.3%) in mechanical valgus: HKAA (171o-178o)
Three failed with infection; 2 after ORIF and one with BMI>50. The greatest post op varus suffered peri-prosthetic fracture. There was no aseptic loosening or instability.
Only full-length radiographs accurately measure alignment and very few similar studies exist. No cases failed by loosening or instability, but PPF followed persistent malalignment. Infection complicated prior ORIF and elevated BMI.
This does not endorse indiscriminate use of mechanically constrained knee prostheses. Lower demand patients with complex arthropathy, especially severe deformity, benefit from fully cemented, non-linked constrained prostheses, with intra-medullary fixation. Hinges are not necessarily indicated, and rotational constraint does not lead to loosening.
