Through the personalized Berger method, the rotational position of the tibial component is achievable for every patient including both TKR and revision arthroplasty.
The optimal rotational position of the tibial component was found to be crucial, as its excessive internal rotation can cause: patellar maltracking, patellofemoral instability, persistent anterior pain, flexion and mid flexion tibiofemoral instability, early wear of the polyethylene (PE) component, early loosening, stiffness and contracture. Excessive internal rotation of the tibial component can cause an extension deficit [1, 3, 9,10,11,12,13, 16,17,18,19, 21, 25, 33, 41, 44, 47, 49].
There is conflicting data relating to the clinically tolerable limits of tibial component internal malrotation. According to Nicoll, the internal rotation error of the tibial component by 8° is tolerable, while Barrack reports 6°; Bell showed that more than 5.8° of internal rotation cause anterior pain of the knee. All of the studies used the classical Berger method [9, 19, 38, 44, 49].
According to Kim, less than 2° of external rotation constitutes a risk of component failure. Malrotation of the components plays a significant role in early TKR failure. Should this cause pain, early revision surgery may become inevitable. Before revision surgery is selected, the cause should be identified. If revision surgery is likely, due to component malrotation - taking into account the limited range of the tolerable angle - an accurate and repeatable measuring method is needed to specify the degree of correction [13, 16, 27, 28, 33,34,35, 46].
Rotation and malrotation of the femoral component are measurable on a single axial CT, referenced to the sTEA. Measurement of the rotation of the tibial component is more complicated, therefore, several methods are recommended, using various anatomical reference points, with the help of 2D and 3D CT.
Usually, the TT, the MBPT or its medial 1/6 or 1/3, the centre of the TT or that of the patellar tendon, are used as the anterior reference point. The anterior tibial curved cortex is used by Baldini, Indelli and Kim. The centre of the PCL, the centre of the greatest mediolateral width of the tibia, the centre of the posterior tibial margin axis, the centre of the sTEA transposed to the tibia or the gCTP, are used as the posterior reference point [4, 5, 7, 8, 12, 14, 20, 22, 23, 26,27,28,29,30,31,32,33,34,35, 44, 48, 49].
The accuracy and the repeatability of 15 different tibial axes were evaluated by Saffarini et al. in a systematic review. The angle between the perpendicular line to the sTEA and the original Akagi line was the most consistent. The methods using the gCTP as the posterior reference point showed the best inter-observer repeatability. However, the minimum and the maximum values of the angulations, compared to the perpendicular line to the sTEA, significantly exceeded the thresholds of clinical relevance. A difference of less than ±5° between the line connecting the gCTP and the medial 1/3 of the TT and the line perpendicular to the sTEA was found in 67.5% of cases; a difference between the line connecting the gCTP and the medial border of the TT and the line perpendicular to the sTEA was found by Lützner in only 3.8% [4, 5, 8, 14, 20, 22, 27, 29,30,31, 35, 39, 43, 44].
Rotation between the femoral and tibial components is restricted by the fixed bearing insert unpredictably, therefore, the angle between the sTEA or the line perpendicular to it, and the axis of the tibial component, should be evaluated critically. In this way, a method should be chosen which uses solely the landmarks of the tibia. The centre of the PCL and the Akagi line, which is seen as a gold standard, are difficult to identify on the 2D CT, following TKR [4, 5, 35].
As a result of the above, the gCTP can be used as the posterior, while certain specified points of the TT or parts of the patellar tendon can be used as the anterior reference points, meeting the recommendations of other authors. The Berger method, which evaluates the rotation of the femoral and tibial components, depends upon the gCTP-TTT distance; therefore its value varies from patient to patient [12].
High variability of the mediolateral localization of the TT was realized by Howell (32-47 mm between the medial border of the TT and the MC of the tibia); 70% of cases were outside the range of the clinically relevant threshold while using the gCTP as the posterior reference point. This means that in the vast majority of cases, the difference was more than the clinically significant threshold. In the case of 45.9% of the patients, the value was ±3° and 70.6% of the population, measured using our method, were within the ±5.8° range [27, 35].
Simply measuring 18 degrees to the line between gCTP and TTT (or CIP) can lead to severe rotational malposition. (Fig. 2A, B).
If the method could be carried out with acceptable reliability and repeatability on the preoperative CT scans, then the degree of the correction, carried out on account of the tibial component malrotation can be defined precisely [6, 8, 12, 20, 23, 25, 26, 29,30,31, 34, 35, 44].
The repeatability and reliability of the classic Berger method and its modifications were examined by several authors. Most of them mention good and excellent intra- and inter-observer intra-class correlation coefficients (ICC) [8, 23, 26, 27, 29,30,31, 35, 42, 44, 48].
Even if most of the authors found good and very good intra and inter-observer correlation, the minimum and the maximum values of the angulations, compared to the perpendicular line to the sTEA, significantly exceeded the thresholds of clinical relevance [8, 14, 20, 22, 35, 44].
The Berger method was improved by us to achieve better repeatability. Following an analysis of our experiences, we concluded that the centre of the TT is difficult to identify accurately on a 2D axial CT, which concurs with the findings of Saffi. The method proposed by us can be implemented in any institute using CT scans; any software that automatically enables the ellipse can optimize the ICC even further [12, 44].
Our results are amongst the best, with similar inter- and intra-observer reliability compared to other authors; for this, we have the following explanations: the reference points were specified in a precise manner and all three of the examiners followed the same protocol [6, 8, 22, 23, 26, 27, 29,30,31, 34, 35, 39, 42, 44, 48].
Concerning our study and the method proposed by us, there are certain limitations. The case number was relatively low with only three examiners. Even so, the multiplicity for statistical analysis was satisfactory. CT scans from knees awaiting TKR were evaluated to obtain the data for our method, required to plan revision arthroplasties. The personalized Berger method was developed on CT scans with metal subtraction of patients awaiting revision arthroplasty. The number of the cases was not satisfactory for the research of reliability in the case of postoperative CT scans; therefore preoperative CT scans were used in further stages of the study.