What did the study look at?
In total knee replacement (total knee arthroplasty), the metal base plate placed on top of the shinbone should sit at the correct angle (rotational alignment) and cover the bone surface as fully as possible. The authors note that an undersized plate may lead to collapse, early loosening and fractures, while an oversized one may cause chronic pain or restricted movement.
Both symmetric and asymmetric base plates are available, and which is better remains debated. The researchers tested two hypotheses:
- When placed in the same rotational axis, the symmetric plate covers the bone better.
- The asymmetric plate leads to more rotational errors.
How was it done?
This was a cadaveric study: it was carried out not on patients but on 80 dry adult tibias. Genesis II was used as the symmetric system and Nex-Gen as the asymmetric system. Two orthopaedic surgeons experienced in arthroplasty performed the placements.
- Part one (40 bones): the surgeons placed 20 symmetric (group 1) and 20 asymmetric (group 2) plates, aiming for the best coverage.
- Part two (40 bones): they placed 20 symmetric (group 3) and 20 asymmetric (group 4) plates, this time aiming for the best rotation.
The bones were then scanned with computed tomography and three-dimensional models were created. A musculoskeletal radiologist blinded to the study's purpose measured the proportion of uncovered bone, the proportion of plate overhanging the bone and the plate's angle relative to a reference line (the posterior condylar axis).
What did it find?
When best coverage was the goal (groups 1 and 2):
- Uncovered bone surface was 3.8% ± 1.8% with the symmetric plate and 4.4% ± 2.6% with the asymmetric plate; the difference was not significant (p = 0.624).
- Overhanging plate area was 2.2% ± 0.4% and 2.7% ± 1.6%; not significant (p = 0.819).
- The rotation angle was 3.1° ± 1.4° in the symmetric group and 7.5° ± 3.2° in the asymmetric group, a significant difference (p = 0.034). All plates were externally rotated.
When best rotation was the goal (groups 3 and 4):
- The rotation angle was 3.4° ± 1.6° and 4.1° ± 2.2°; not significant (p = 0.36).
- Uncovered surface was 2.9% ± 1.7% with the symmetric plate and 6.7% ± 3.1% with the asymmetric plate (p = 0.041).
- Overhanging plate area was 1.9% ± 0.6% and 4.3% ± 2.4% (p = 0.029).
The authors interpreted this as showing that small rotational adjustments made to increase coverage with the asymmetric plate can result in rotational errors.
The authors' conclusion
According to the authors, placing the tibial component at the correct size and rotation is essential for a good outcome after total knee replacement. In this study, the symmetric base plate design was more effective than the asymmetric design in achieving ideal tibial rotation and coverage.
Limitations
The authors list as limitations that this was a cadaveric study and that the number of tibias was relatively small. The findings do not directly show clinical outcomes of operations in living patients.
Reference
Bozkurt M, Akkaya M, Tahta M, Gürsoy S, Firat A. 2017. Tibial Base Plate for Total Knee Arthroplasty: Symmetric or Asymmetric? Clinics in Orthopedic Surgery 9(3):280-285. Licence: CC BY-NC 4.0.
DOI: 10.4055/cios.2017.9.3.280
Free full text: PubMed Central
This article summarises a published scientific paper for general information; it does not replace an examination or a personal medical assessment. Please consult your doctor about your symptoms.
