Introduction

The main principle of correction osteotomies is to achieve a transfer of loading from diseased, arthritic areas of the joint to areas with relatively intact, healthy cartilage. The valgus opening-wedge high tibial osteotomy (HTO) is an option for the treatment of medial varus osteoarthritis of the knee that aims to change the loading of the tibiofemoral joint by a correction of the leg axis. The degenerated medial compartment cartilage is thus decompressed, resulting in a relief of pain and a delay of cartilage damage.

Good clinical short- and mid-term results have been reported.¹^–⁸ However, favorable results seem to be strongly dependent on a precise correction of the loading axis.⁹ Under-correction with persisting varus usually leads to poor results; overcorrection into large valgus may result in medial joint opening and rapid development of lateral osteoarthritis.¹⁰ Various recommendations on the precise amount of the valgus alignment achieved postoperatively have been published. Some authors refer to the anatomical axes of the femur and tibia and recommend a postoperative alignment of 8 to 10 degrees valgus.¹^–¹⁴ Others focus on the mechanical axes of the femur and tibia and suggest a postoperative alignment of 3 to 5 degrees valgus.^5,^10,^15,¹⁶ However, these suggestions are based on individual clinical experiences and a few retrospective studies.

Probably the most commonly applied concept in preoperative planning is based on the work of Fujisawa et al.¹⁴ In this retrospective clinical study, preoperative and postoperative arthroscopy with a follow-up for 4 months to 6 years was performed on 54 knee joints before closing-wedge HTO, and the tibiofemoral joint cartilage was evaluated. The postoperative weight-bearing axis was correlated with changes in cartilage findings. The correction achieved postoperatively was measured with reference to the intersection of the weight-bearing axis with the tibial plateau. The tibial plateau was divided into four sections and the point of intersection assigned to the corresponding section. Fujisawa et al.¹⁴ concluded from their findings that for ideal correction the postoperative axis should lie within the lateral 30% of the tibial plateau as measured from its midpoint. From this study derived the recommendation that the weight-bearing axis should postoperatively pass through the “Fujisawa point” (i.e., it should intersect with the knee joint line at 62% of the tibial plateau width measured from its medial cortex).¹⁷^–¹⁹

PEARLS

Principles of Osteotomy

• Goal of valgus opening-wedge high tibial osteotomy: redistribution of pressure on the tibiofemoral joint cartilage.

• Good results are achieved with high tibial osteotomy if correction is precise.

• Different statements are made in the literature on the necessary amount of correction and on the aimed weight-bearing axis after correction.

Effect of Different Load Axes on Tibiofemoral Cartilage

Literature

Surprisingly, despite the enormous increase of interest in osteotomies, to date no investigations have been reported that quantified the load-transfer effect of shifting the weight-bearing line on tibiofemoral joint contact pressure with reliable implication for the right amount of correction. The work of Fujisawa was undertaken as early as 1979, when knee arthroscopy, the only parameter used in the study to evaluate outcomes, was still developing. Further weaknesses of that study are a relatively small sample size per patient group (approximately 12 knee joints) and a complete lack of postoperative clinical data such as pain or joint effusion as parameters for good outcome.

Because of the discrepancy between the high number of osteotomy procedures being performed and the lack of experimental data on the actual intraarticular effects of correction osteotomies of the knee, the authors undertook a literature search on this subject and carried out the experimental study that will be presented in this chapter.

The effects of different load axes in the frontal plane on the distribution of pressure in the tibiofemoral compartments has already been the subject of several experimental studies with different measuring techniques and experimental designs.

Before direct measurement of joint cartilage pressure was technically feasible, indirect methods were employed based on evaluation of radiographs, eventually combined with gait analysis and measurements of ground reaction forces with force plates.²⁰^–²²

The first direct measurements of transferred joint forces and contact pressures were obtained by application of strain gauges.^23,²⁴ Next, pressure-sensitive films (Fuji film) were used that changed color as a result of applied pressure.^25,²⁶

The up-to-date techniques use pressure-sensitive film that measures changes in electromechanical resistance (K-Scan System, Tekscan, Inc., Boston, Mass.). These sensors offer the highest accuracy and best reproducibility and have provided the basis for the most recent studies on this subject.²⁷

Most studies are tested under static conditions and rarely under dynamic conditions (i.e., simulation of gait cycle with an increase and decrease in loading). Nevertheless, all the studies were basically able to demonstrate that alterations of the axial alignment led to redistribution of load within the compartments of the tibiofemoral joint. Varus deformity resulted in greater loading of the medial compartment and valgus deformity in greater loading of the lateral compartment. This inequality in the distribution increased with the degree of axis deviation, proximity to the joint line, and amount of applied force.^24,²⁵

In addition to those studies describing tibiofemoral pressure distribution under the influence of different load axes in the frontal plane, only two experimental studies have been published that investigated the effect of high-tibial-correction osteotomy on contact pressures distributions within the knee joint. Only one of these studies was concerned with the correction of deformities in the frontal plane (valgus-varus)²⁶; using the closing-wedge technique. The other study, published by the present author, investigated contact pressure changes after correction osteotomy in the sagittal plane.²⁷ Measurements of the contact pressure in the knee joint after correction osteotomy in the varus-valgus plane in open-wedge technique have not been published so far. Furthermore, data correlating the degree of valgus with the desired changes in intraarticular contact pressure distribution are currently not available.

In a biomechanical study with knee joint specimens, Riegger-Krugh et al.²⁵ investigated the effect of HTO in closing-wedge technique on the contact pressure distribution within the tibiofemoral joint. Fuji films were used for pressure measurement (i.e., only measurement under static conditions were possible and there were no repeated measurements). Both varus and valgus malalignments were simulated and a closing-wedge osteotomy of 5 degrees was carried out. The authors found differences in contact pressure distribution as expected, with higher contact pressure in the medial compartment in varus malalignment and higher contact pressure in the lateral compartment in valgus malalignment. Surprisingly, with a neutral loading axis, less contact pressure was recorded in the medial compartments than in the lateral compartments under these test conditions; after valgus HTO, the contact pressure distribution was similar to that with neutral loading axis. The authors concluded that because of those only minimal changes in the contact pressure distribution, an HTO of 5 degrees would represent in most patients an under-correction.

Regarding the experimental setup, general conclusion for the amount of correction in the clinical situation can only be drawn to a limited extent. The mounting device used for the specimens allowed only for limited adaptations to the different preexisting anatomical axes and therefore did not simulate natural conditions.

In a biomechanical study conducted by the authors,²⁷ tibiofemoral contact pressure was measured after flexion HTO (i.e., after altering the mechanical axes in the sagittal plane). In a dynamic experimental model involving a knee joint kinemator, contact pressure was measured with the K-Scan System. Flexion osteotomy was performed as a supra tuberositous opening-wedge osteotomy parallel to the tibial slope of the tibial plateau, and then the tibial slope was gradually increased in 5 degrees increments from its original position to 20 degrees. This osteotomy exclusively modified the alignment in the sagittal plane; the alignment in the frontal plane (varus-valgus) was left unchanged.

The flexion osteotomy significantly altered the tibiofemoral contact pressure distribution: as the tibial slope increased, the tibiofemoral contact area in the medial compartment shifted anteriorly, whereas the posterior joint area was unloaded. This effect was most prominent in extension (i.e., in a position with high loading).

In addition to these biomechanical studies with cadaveric specimens, two other studies have been published that measured tibiofemoral contact pressure intraoperatively in vivo. One study used a knee prosthesis with an implemented telemetric pressure measurement device during the procedure of total knee replacement with the patient supine under general anesthesia, followed by testing in normal alignment and under varus and valgus stress produced by the manipulations of the surgeons. The second study measured tibiofemoral contact pressure during diagnostic arthroscopy with the patient under local anesthesia. F-scan pressure measurement sensors from the K-Scan System were inserted into the medial and lateral compartments, and contact pressure was recorded during two-legged and single leg stance and after application of a valgus splint.