LIVRE JDG 2010

DISCUSSION

Medial UKAs were performed for patients

with varus malalignement for any etiology

(arthritis or necrosis). As recommended in the

literature [5, 20, 21], we tried to conserve a

slight hypocorrection. However, some authors

[11, 18] tend to avoid hypocorrection because

of a risk of early wear. We did not have wear

related to that etiology. We found a slight val-

gus in the lateral UKA but it was not systema-

tic with some tibial implant measured in varus.

It may be explained by the origin of the defor-

mity: varus deformity tends to be mainly tibial,

but valgus deformity is usually mixed. It is

therefore important to perform a tibial cut at

nearly 90° in case of lateral UKA because the

surgeon does not want to perform a cut in val-

gus for a lateral UKA [21].

The mean tibial slope averaged 5°. For an

UKA we attempt to reproduce the tibial slope

of each individual. Hernigou

et al.

[10] sho-

wed the tibial slope had to be under 7°.

Beyond this point, there was a greater risk of

failure by increasing the anterior tibial transla-

tion and further rupture of the anterior crucia-

te ligament. Whiteside and Amador [25]

recommended a tibial slope between 3° and 7°,

whereas Matsuda

et al.

[13] found a tibial

slope averaging 10.7° for the medial tibial pla-

teau and 7.2° for the lateral tibial plateau.

Dejour and Bonnin [6] measured a mean

radiological tibial slope of 10° in the normal

population, but they highlighted also that a 10°

increase of the tibial slope would lead to

3.5mm increase of the anterior tibial transla-

tion. That increase could lead to rupture of the

anterior cruciate ligament and to greater bio-

mechanical constraint of the posterior part of

tibial component.

Regarding our rotation measurements, we may

point out the choice of the landmarks refe-

rences. For total knee arthroplasty, the tibial

component rotation has been analyzed several

times [22, 23]. At the proximal tibial, many

references have been used: the transepicondy-

lar axis, the medial border of the anterior tibial

tubercle, the center part of the patellar tendon,

the tibial insertion of the posterior cruciate

ligament (PCL), the medial and lateral parts of

the tibial plateaus and the posterior parts of the

tibial plateaus. Our choice (posterior parts of

the tibial plateaus) from Yoshioka [27] may be

criticized. Nevertheless there is a high variabi-

lity with the PCL tibial insertion [22] which is

also difficult to locate on a CT scan. The UKA

implant positioning in rotation is still challen-

ging and there is little scientific support to

assess the proper position. The tibial implant

rotation is guided by the antero-posterior cut.

It is sometimes said that the cut should be done

with the knee flexed at 90° and that the sur-

geon should aim the saw towards the hip. We

may accept the approximate feature of that cut.

In our series, the tibial implant was externally

rotated for both medial and lateral UKA with

no involvement of the femoral valgus. We

found only one study with analysis of the tibial

plateau positioning on the axial plane.

Campbell

et al.

[4] report great variation of the

tibial component orientation but he did not

give its entire data for the rotation in his

article. At last the CT-scan allowed to analyze

the tibial implant positioning on the axial

plane and may be to highlight a rotation mal-

positioning (fig. 3). The biomechanical conse-

quences of such malpositioning have never

been studied and we may find out a hidden

etiology of ongoing pain or early failure of an

UKA.

JOURNÉES LYONNAISES DE CHIRURGIE DU GENOU

176

Fig. 3: Lateral UKA with an excessive rota-

tion of the tibial plateau.