11 Osteotomy and ligament instability: tibial slope corrections and combined procedures around the knee joint
High-tibial osteotomy (HTO) has become an established method for treatment of medial unicompartmental osteoarthritis of the knee by correction of varus alignment in the frontal plane. Transferring the mechanical axis from the degenerated medial joint compartment to the lateral compartment results in medial decompression, thus reducing pain and also delaying the progression of osteoarthritis.
Another indication for valgization HTO is treatment of patients with varus deformity and simultaneous chronic knee instability. Posterolateral instability with varus deformity results in lateral joint subluxation during weigth-bearing (known as varus thrust). In these patients, valgization osteotomy leads indirectly to joint stabilization since the shift of the mechanical axis to the lateral compartment eliminates posterolateral hyperdistraction.
As described in chapter 9 “High-tibial open-wedge valgization osteotomy with plate fixator”, HTO for varus joint degeneration only aims for correction in the frontal plane (varus/valgus), whereas the sagittal plane (flexion/extension) remains unconsidered. Anatomically, the tibial plateau is not perpendicular to the tibial shaft axis, but the plateau declines caudally (tibial slope) at an angle of about 9–11° medially and 6–8° laterally with a large range of variation. It seems reasonable to assume that changes of the tibial slope will substantially affect knee biomechanics. In veterinary medicine it has been shown that anterior subluxation of the tibia in canine caused by a rupture of the anterior cruciate ligament can be treated by extension osteotomy (decreasing the tibial slope).
However, these anatomical and biomechanical conditions completely differ from human beings. Experimental studies investigating the exact relationships between tibial slope, sagittal translation of the tibia, ligament stability, and range of motion are currently lacking.
Physiological: the slope of the tibial plateau is caudally inclined in relation to the horizontal plane.
The slope is measured on the lateral x-ray with a long view of the tibial shaft. The tangent is marked at the medial and lateral tibial plateau.
Definition of slope: angle between the tangents at the tibial plateau and the anatomical tibial axis minus 90° (see Fig 11-11 ).
The technique of medial open-wedge tibial osteotomy allows for alteration of the sagittal inclination of the tibial head by eccentric distraction of the osteotomy gap. If the osteotomy is opened anteriorly more than posteriorly, the tibial slope increases (flexion osteotomy); if it is opened posteriorly more than anteriorly, the slope decreases (extension osteotomy).
This chapter illustrates the effect of changes of the tibial slope on the biomechanics of the knee joint based on a biomechanical study conducted by the authors.
2 Biomechanical study with human cadaver knees
High-tibial flexion osteotomy cranial of the tibial tuberosity was performed on seven human cadaveric knee joint specimens without osteoarthritis or previous operations and with intact ligaments. The osteotomy was performed anteriorly in open-wedge technique, calibrated plastic wedges were inserted into the osteotomy gap from ventral in order to achieve standardized opening. The tibial slope was gradually raised from its original status (10° ± 4°) to 20° in steps of 5°. The osteotomy was stabilized with an external fixator ( Fig 11-1 ).
A special computer-assisted test system (knee kinemator) was used to simulate physiological knee joint movements ( Fig 11-1, Fig 11-2 ). An isokinetic extension movement between 120° flexion and full extension was achieved by tensioning the quadriceps tendon and applying an antagonistic force to the tibial shaft. Special suspension attachment of the tibia allowed for an unconstraint motion of the tibia on the femoral condyles, ie, the proximal tibia was flexible in all planes (extension/flexion, varus/valgus, rotation, sagittal translation) and only restricted by the bony anatomy and the ligaments.
Joint kinematics were recorded using a specially designed three-dimensional ultrasound tracking system (Zebris), whereby parameters such as sagittal translation and rotation of the tibia in relation to the femur were analyzed with special software to an accuracy of 0.1 mm and 0.1°. A measuring sensor was inserted into the anteromedial bundle of the anterior cruciate ligament (ACL) to record any changes of its tension allowing for a continuously monitoring of ligament tension during the extension movement. Tibiofemoral cartilage pressure in the medial compartment was also evaluated (Tekscan sensors, also see chapter 10 “Effect of osteotomies on cartilage pressure in the knee”).
The experiments were performed with intact ligaments and after transection of the posterior cruciate ligament (PCL) to simulate knee instability.
2.1 Results of the biomechanical studies
Between full extension and 60° knee flexion sagittal translation of the tibia (drawer phenomenon) was found to be highly significantly dependent on the degree of inclination of the tibia plateau (tibial slope). In comparison to the data obtained with the physiological slope, flexion osteotomy resulted in an anterior translation of the tibial head of up to 7 mm. The posterior drawer that occurred after transection of the posterior cruciate ligament was completely reduced by an elevation of the tibial slope of 5° and was even inverted into an anterior drawer ( Fig 11-3 ).
No significant changes after slope alteration were identified for tibial rotation, especially the final rotation movement of the tibia (external rotation of the tibia at the end of the extension movement) remained unaffected ( Fig 11-4 ) as well as movements in the varus-valgus plane.
It would seem reasonable to assume that an increased anterior translation of the tibial head after slope elevation would lead to increased tension of the ACL. In this study the tension of the anterior cruciate ligament remained relatively constant during slope elevation to 10°. Minor increase occurred at slope elevations of 15° or 20° ( Fig 11-5 ).
This fact can be explained by the surgical technique since the described opening procedure raises the proximal tibia in the region of the tibial insertion of ACL and thus relaxes the ligament.
Analysis of cartilage pressure showed that an increase of the tibial slope shifted the tibiofemoral contact area anteriorly ( Fig 11-6, Fig 11-7 ), which is due to an earlier contact of the tibial plateau on the femoral condyle after slope elevation. The medial femoral condyle is moved anteriorly during tibial slope elevation leading to decompression of the posterior cartilaginous areas of the plateau and posterior horn of the medial meniscus ( Fig 11-8 ).
Summary and conclusions based on biomechanical results
Alteration of the inclination of the tibial head (tibial slope) in open-wedge osteotomy affects the kinematics of the knee joint. Therefore, the tibial slope should not be increased or decreased during osteotomy in the frontal plane (valgus or varus correction) in patients with stable ligaments and normal range of motion.
Increase of the tibial slope shifts the tibial head anteriorly in relation to the femoral condyle. For example, in PCL insufficiency the tibial head can be reduced from the posterior drawer by increasing of the tibial slope. Alternatively, osteotomy with decrease of the tibial inclination can be applied in patients with anterior instability to reduce anterior subluxation of the tibia.
Increase of ACL tension during flexion osteotomy only occurs at slope elevation of more than 10°.
Increase of the tibial slope leads to anterior shift of the contact area between the femur and the tibia in extension. The greater the slope, the higher the load on the cartilage of the anterior half of the tibial plateau with simultaneous unloading of the posterior half. If there is cartilage damage of the posteromedial tibial plateau, eg, after partial resection of the posterior horn of the medial meniscus, osteotomy with slope elevation may selectively unload those damaged cartilaginous areas.