5 Detailed planning algorithm for high-tibial osteotomy
Valgization osteotomy of the proximal tibia remains the treatment of choice for the young active patient with a progressively symptomatic varus knee and mild to moderate osteoarthritis. Although the natural history of the varus knee is not well established, it is widely accepted that patients with varus malalignment are prone to develop more severe medial compartment osteoarthritis unless the normal mechanics of the knee are restored [1–3]. Careful preoperative planning is mandatory to avoid undercorrection or overcorrection, two factors held responsible for early failure of the procedure [4–6].
Different planning methods may use different definitions for joint center, axis, and weight-bearing line.
For the definitions of physiological axes and angles of the lower limb see chapter 1 “Physiological axes of the lower limb”.
The following planning methods are applicable for deformities in and near the knee joint in the absence of gross angular deformities of the long bones due to fracture pseudarthrosis or osteomalacia.
2 Radiographic work-up
Preoperative radiographic evaluation for high-tibial osteotomy (HTO) includes bilateral weight-bearing AP views in full extension and bilateral weight-bearing (PA) tunnel views in 45° of flexion (Rosenberg view, ). These allow an assessment of both the extent of knee arthritis and lower extremity alignment. Lateral and skyline views of the involved knee are also obtained. The planning should be performed on a long-leg weight-bearing AP x-ray including the hip and ankle joint obtained in correct rotation of the knee with the patient loading both legs. The authors’ preferred planning method is based on this long-leg x-ray.
In clinical settings where hip and ankle joint centers cannot be simultaneously radiographed (“short AP view”), the exposed part of the distal femur and proximal tibia should be as long as possible. This allows for accurate measurements using the anatomic axis of both femur and tibia. Proper rotation of the limb is important and requires the patella to be centered between the femoral condyles and directed forward in the AP projection. Standardized technique is mandatory to assure that the x-rays are reproducible. In cases with a significant extension deficit, separate x-rays of the femur and tibia need to be obtained to enable accurate planning. Soft-tissue laxity can be quantified by the amount of medial joint opening between the involved and contralateral legs on the Rosenberg view  ( Fig 5-1 ).
3 Etiology of varus alignment
Total varus angulation of the knee is the sum of three potential components :
The femorotibial geometric alignment
Narrowing of the medial joint space due to wear and tear of the meniscus and the osteocartilaginous complex
Separation of the lateral joint space due to lax lateral soft tissues and ligaments
The femorotibial alignment can be quantified by the mechanical or anatomical angle as mentioned above. Wear and tear of the medial compartment with consecutive joint-space narrowing on x-rays can be quantified by a number of different classification systems [9–11]. The amount of varus angular deformity caused by lateral joint opening must be compensated when planning an osteotomy to avoid an overcorrection.
3.1 Mathematical method
The side-to-side difference in lateral joint separation can be measured on the Rosenberg view ( Fig 5-1 ). The following equation defines the amount of increased varus angulation resulting from a given separation of the lateral tibiofemoral joint due to slack lateral restraints (soft-tissue laxity) :
Where ΔS equals the increments of lateral joint separation as a side-to-side difference, TW equals the tibial width, and c being a named constant of 76.4. The following case illustrates the equation: A 40-year-old male patient with progressive right medial knee pain has a lateral joint opening on varus testing following a twisting injury to his knee. A weight-bearing full-length x-ray shows a varus angular deformity of 7°. The Rosenberg view displays a 4 mm increase in lateral joint opening ( Fig 5-1 ). The tibial plateau measures 80 mm in width. In this case, the calculated amount of varus angular deformity caused by slack lateral soft-tissue restraints is
Failure to account for the soft-tissue component of lateral joint opening would result in approximately 3.8° of valgus overcorrection .
3.2 Graphical method
In addition to the mathematical approach to quantify an eccentric joint space, there is a graphic alternative that might be more suitable for clinical practice. The tibia is redrawn and copied on glassine paper with the original x-ray underneath. After compensating the eccentric joint line the tibia is then fixed in the new position and the required degree of correction can easily be measured.
4 The aim of high-tibial osteotomy
The optimal correction and preoperative planning method recommended for varus knee deformities varies according to the author ( Table 5-1 ). Both mechanical or anatomical femorotibial angles and the weight-bearing line can be used to plan the surgery. Coventry  recommended overcorrection of the varus alignment to at least 8° of anatomic femorotibial valgus based on regression analysis of the longevity of high-tibial osteotomies. Hernigou  used the mechanical limb axis and found good clinical results in patients with a mechanical valgus angle between 3-6°. Smaller (< 3°) or greater (> 6°) correction angles were associated with poorer clinical results. Recently, Dugdale and Noyes  showed that correcting an angular deformity based on the weight-bearing line (WBL) ( Fig 5-2, Fig 5-3 ) accounts for tibial and femoral length and is more accurate than relying on the femorotibial angle as determined from limited x-rays. They recommended aiming for a postoperative WBL, cutting the proximal tibia on a point 62-66% of the tibial width in the frontal plane. This point usually corresponds to the lateral inclination of the lateral tibial spine and to a mechanical femorotibial valgus angle of 3-5°. The preference to use the WBL for planning an osteotomy is based on the study by Fujisawa  who demonstrated that cartilage ulceration did not further deteriorate after HTO in cases where the WBL passed through the optimum zone. He divided the tibia in two halfs and defined this optimum zone as 30-40% width of the lateral half of the plateau. This corresponds to the recommendations of Miniaci  (60-70% width of the tibial plateau) and Noyes  (62% width of the plateau) since these authors based their calculations on the entire width of the tibia as 100% and not on the lateral half of the plateau as Fujisawa did ( Fig 5-2 ).
Use long-leg weight-bearing x-rays if possible.
Assure symmetrical weight bearing and correct rotation for planning x-rays.
Use weight-bearing line as base of planning, if possible.
In valgization HTO the postoperative weight-bearing line should pass through a point 62% of the width of the tibial plateau on the lateral side, usually corresponding to the down slope of the lateral spine.
In cases with asymmetric joint opening under weight bearing, the divergence of the femoral and tibial joint lines must be compensated either mathematically or graphically.