10 Effect of osteotomies on cartilage pressure in the knee
The aim of correction osteotomies is the transfer of mechanical load from diseased, arthritic areas of the joint to areas with intact, healthy cartilage. The open-wedge high-tibial valgization osteotomy (HTO) is a treatment method for medial varus osteoarthritis of the knee aiming for a shift of the weight-bearing axis 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 precisely performed correction of the mechanical axis . Undercorrection with a persisting varus usually leads to poor results. Overcorrection into large valgus may result in medial joint opening and rapid development of lateral osteoarthritis . 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 8–10° valgus [11–14]. Others focus on the mechanical axes of the femur and tibia and suggest a postoperative alignment of 3–5° valgus [5, 10, 15]. However, these recommendations are based on individual clinical experiences and only a few retrospective studies.
Principles of correction osteotomy
The aim of open-wedge high-tibial valgization osteotomy is the redistribution of pressure on the tibiofemoral joint cartilage.
Good results are achieved with high-tibial osteotomy if correction is precisely performed.
Different recommendations regarding the amount of correction are found in the literature.
Probably the most commonly applied concept in preoperative planning is based on the work of Fujisawa et al . In this retrospective clinical study, arthroscopy was performed in 54 knee joints before and after closed-wedge HTO evaluating the tibiofemoral joint cartilage with a follow-up of 4 months to 6 years. The postoperative weight-bearing axis was correlated with cartilage damages. The correction achieved postoperatively was evaluated by measuring the intersection of the corrected weight bearing axis with the tibial plateau. The tibial plateau was divided into four sections and the point of intersection was assigned to the corresponding section. Fujisawa et al  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 emerged the recommendation that the weight-bearing axis should postoperatively pass through the so-called “Fujisawa point”, ie, it should intersect with the knee joint line at 62% of the tibial plateau entire width measured from its medial cortex [17–19].
Despite an enormous increase of interest in osteotomies, so far no results have been published that quantify the load-transfer effect of shifting the weight-bearing axis on tibiofemoral joint contact pressure with reliable implication for the right amount of correction. The work of Fujisawa et al was published in 1979, when knee arthroscopy, the only parameter used in the study to evaluate outcomes, was still developing. Further weaknesses of the mentioned study are the relatively small patient groups (approximately twelve knee joints per group) and the complete lack of postoperative clinical data such as pain or joint effusion.
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 a biomechanical study that will be presented in this chapter.
2 Literature overview: experimental studies of the effect of different load axes on tibiofemoral pressure distribution
The effect of different loading 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 possible, indirect methods were employed based on evaluation of x-rays, eventually combined with gait analysis and measurements of ground reaction forces with force plates [20–22]. The first direct measurement of transferred joint forces and contact pressure was obtained by application of strain gauges [23, 24]. Next, pressure sensitive films (Fuji films) were used, indicating the applied pressure by change of colour [25, 26]. The current techniques employ pressure-sensitive film that measures changes in electromechanical resistance (Tekscan system). These sensors offer highest accuracy and best reproducibility and have provided the basis for the most recent studies on this subject [27, 28].
In the past, the majority of studies were based on test cycles under static conditions and rarely under dynamic conditions, ie, 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 resulted in 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 and the amount of applied force [24, 25].
In addition to those studies describing tibiofemoral pressure distribution under the influence of different loading axes in the frontal plane, only two experimental studies have been published on the effect of high-tibial correction osteotomy on contact pressure distribution within the knee joint. One of these studies employed the closed-wedge technique for correction of deformities in the frontal plane (valgus-varus) . The other study, published by the authors, focused on contact pressure changes after correction osteotomy in the sagittal plane . Measurements of the contact pressure in the knee joint after correction osteotomy in the varus-valgus plane in open-wedge technique have not yet been published. 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  examined the effect of closed-wedge HTO on contact pressure distribution in the tibiofemoral joint using Fuji films. However, in this study measurements were only possible under static conditions. Both varus and valgus malalignment were simulated and a closed-wedge osteotomy of 5° was performed. 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 neutral loading axis less contact pressure was recorded medial than lateral. After valgization HTO the contact pressure distribution was similar to that of a neutral leg axis. The authors concluded that because of those minimal changes in the contact pressure distribution a closed-wedge HTO of 5° would result in an undercorrection in most patients.
Since the used mounting device for the specimens allowed only for limited adaptations to the different preexisting anatomical axes and did not simulate in vivo conditions, general conclusions for the degree of correction in the clinical situation can only be drawn to a limited extent.
In a biomechanical study conducted by the authors tibiofemoral contact pressure was measured after flexion HTO, ie, 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 Tekscan system. Flexion osteotomy was performed as a supratuberositous open-wedge osteotomy parallel to the slope of the tibial plateau and then the tibial slope was gradually increased from its original position in 5°-steps to 20°. This osteotomy addressed the alignment in the sagittal plane. The alignment in the frontal plane (varus-valgus) was not changed.
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 (see chapter 11 “Osteotomy and ligament instability: tibial slope corrections and combined procedures around the knee joint”).
In addition to these biomechanical studies on cadaveric specimens, two other studies that measured tibiofemoral contact pressure intraoperatively in vivo have been published. One study measured pressure distribution with an implemented telemetric pressure measurement device during implantation of a total knee replacement. The patient was supine under general anesthesia, testing was performed in normal alignment and under varus and valgus stress . The second study measured tibiofemoral contact pressure during diagnostic arthroscopy with the patient under local anesthesia. F-scan pressure measurement sensors from the Tekscan system were inserted into the medial and lateral compartments and contact pressure was recorded during two-leg and single-leg stance and after application of a valgus splint .
Literature overview of cartilage pressure measurements
Early studies: indirect measurement of intraarticular pressure by analysis of x-rays, gait analysis, and measurement of ground reaction forces.
Direct measurement of pressure on the cartilage for the first time with color-coded films (Fuji): different contact pressure distribution due to different mechanical axes.
Only one study published on cartilage pressure measurement after hightibial valgization osteotomy in closed-wedge technique .
No study published on valgization osteotomy in open-wedge technique.