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TKA patients at risk for pseudo-patella baja
Study finds 26.7% of TKA patients developed a pseudo-patella baja, rising to 47% with extensive soft-tissue release and thick tibial inserts.
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Using extensive soft-tissue balancing and thick tibial inserts during total knee arthroplasty significantly raises the risk of a pseudo-patella baja. But a recent study shows no correlation between pseudo-patella baja and changes in clinical outcome.
Paul Thornton-Bott, MD, of Stoke Mandeville Hospital in the United Kingdom, presented the study at the British Orthopaedic Association Annual Congress, showing that pseudo-patella baja occurred in 27.3% of all total knee arthroplasty (TKA) patients when measured with the Caton-Deschamps method.
“Total knee arthroplasty undergoing extensive soft-tissue release increases the risk of a pseudo baja by 100%,” Thornton-Bott said.
Pseudo-patella baja often occurs after TKA because the tibial prosthesis and insert are thicker than the resected tibia, the researchers wrote in the abstract. “Extensive soft tissue balances may be necessary to correct unbalanced knees, especially in deformed knees,” Thornton-Bott said. “And they may well require thicker tibial inserts.”
However, increasing the tibial insert thickness displaces the tibia distally and reduces the height of the patella from the joint line, potentially increasing tension in the extensor mechanism and causing a potential pathological problem.
Release extent, insert thickness
The prospective multicenter study involved 322 Kinemax TKAs performed by seven surgeons in five centers from October 1999 to December 2002.
An independent observer measured preoperative and postoperative lateral radiographs using the Caton-Deschamps method to assess the patella position. The mean Caton-Deschamps method ratio was 0.715 at preop measurement and 0.457 at postop measurement (P=.028).
Researchers excluded 56 patients with preop baja, which left 266 patients with a normal patella position. They recorded the patients’ soft tissue release extent and tibial insert size.
To determine the impact of extensive soft-tissue releases, the researchers split the patients into three groups of minimal releases, moderate releases and extensive releases.
Of those patients in the minimal and moderate release groups, 21% to 22% were found to have pseudo-patella baja. In contrast, 47% of the extensive release group patients had pseudo-patella baja.
Patients were further split into three groups to determine the effect of insert thickness on pseudo-patella baja. Small inserts were around 8 mm, medium inserts were 10 mm to 12 mm and large inserts were around 15 mm to 22 mm. Researchers found that about 25% of patients in the small and medium insert group had developed a pseudo-patella baja, while 45% to 46% of the large insert group patients had a pseudo-patella baja.
No detrimental effects
Finally, researchers clinically assessed patients with the Oxford Knee Score, the American Knee Society Clinical Rating System, functional outcome scores and range of motion at a minimum 12-month follow-up.
They found no association between a pseudo-patella baja incidence and changes in clinical outcome. “We couldn’t find any correlation at 12 months between any detrimental effects and the knees with pseudo baja,” Thornton-Bott said.
Researchers anticipate that those patients with pseudo-patella baja, however, may also develop true patella baja, as previously reported following TKA, and so they’ve planned a follow-up study with three-year results, Thornton-Bott said.
Shortening of the patella tendon, as measured with the Insall-Salvati method, causes true patella baja. “If, however, the lowering of the patella is due to uneven femoral and tibial cuts … or in our case, soft tissue balancing, then this is called a pseudo-patella baja,” Thornton-Bott said. “It really does not affect the length of the tendon.”
Both true and pseudo-patella baja can cause patella pain and pathology, he said.
For more information:
- Thornton-Bott PJ, Unitt L, Johnstone DJ, Sambatakakis A, et al. Pseudo-patella baja following soft tissue balancing in total knee arthroplasty. Presented at the British Orthopaedic Association Annual Congress. Sept. 20-23, 2005. Birmingham, England.