Although its role was elucidated several years ago, the posterior tibial slope and its relationship to ACL injury and instability have become increasingly recognized in importance more recently. A steep posterior tibial slope, especially in the lateral side, is an accepted risk factor for noncontact ACL injury, both in males, females, and pediatric and adolescent populations [15, 23, 48, 55]. Despite a cutoff value for “at-risk” conditions has not been universally established also due to the different measurement methods, a value >12° is generally considered pathological [24]. Recently, the role of posterior slope has been questioned by Blanke et al. [5]. They reported no significant differences in bony anatomical features of nontraumatic ACL-injured patients compared to a control population. However, the latter study was performed on recreational alpine skiers, thus representing a select subpopulation which may exhibit injury mechanisms that are different from the general population.

The biomechanical background of the correlation between slope and ACL injury relies on the evidence that on the posterior tibial slope, an impulsive compression force (i.e., increased vertical ground-reaction force) during landing generates an anterior shear force [21]. Therefore, a greater posterior tibial slope increases this anterior tibial shear force [37], anterior tibial acceleration and translation, and ACL strain during jump landing activities [28, 35]. Moreover, Dejour and Bonin [16, [34]] in a clinical series demonstrated that increased anterior tibial translation on monopodal stance views correlated with increased posterior tibial slope in patients with intact ACL as well as in those with chronic anterior laxity. Slope has been proposed to play a role also in rotational laxity, as Song et al. [47] correlated a high pivot-shift grade in ACL-deficient patients with the time from injury, anterolateral capsule disruption, lateral meniscus lesion, and also with lateral posterior slope >10.6° measured on MRI according to Hudek et al. [26]. A rotational laxity is also found in patients diagnosed with a so-called bony pivot shift [47] caused by a malunited lateral tibial plateau fracture. The increased slope in the depressed lateral plateau causes symptoms of ACL deficiency in the presence of an intact ACL (Fig. 46.4 bony pivot shift).

The unfavorable effect of an increased posterior tibial slope has been shown to be a risk factor also for revision ACL reconstruction [10, 53]. Webb et al. [53] followed prospectively 181 patients after ACL reconstruction, reporting a significant difference between the radiographically measured medial tibial slope of patients with intact ACL graft (8.5°) and those with both reinjury and contralateral injury (12.9°). The authors quantified the risk of further ACL injury as fivefold compared to patients with medial tibial slope <12°, as this event was reported in 59 % of patients with a value >12°. Similarly, Christensen et al. [10] analyzed the MRI features of 35 patients with successful ACL reconstruction and 35 patients with early graft failure. They reported a higher lateral slope in patients with failed reconstruction (8.4°) compared to intact graft (6.5°) and estimated an odd ratio for graft failure of 1.6, 2.4, and 3.8 for a lateral slope increase of 2°, 4°, and 6°, respectively (Fig. 46.5, failed ACLR).

Thus, in case of failed ACL reconstruction with a posterior tibial slope >10–13° and no evidence of technical errors of the previous reconstructions, a corrective osteotomy could be considered as an option to restore a correct knee biomechanics and avoid further failures.

There are various scenarios of ACL insufficiency that could benefit from a corrective osteotomy, with or without combined ACL reconstruction:

Patient selection is one of the most important factors that determine outcome from surgery. The surgeon must determine whether he or she is suffering from underlying instability or if the complaints are caused by degenerative joint disease. The surgeon can differentiate between the two by determining which activities cause symptoms. It is important to distinguish whether the patient is complaining of pain with aggressive activities and pivoting types of movement, indicating instability, or of pain with activities of daily living, indicating arthrosis.

In the setting of a younger patient who is experiencing symptoms of instability with underlying malalignment and other meniscal or chondral pathology, the surgeon could consider ACL reconstruction in addition to an osteotomy. Surgeons are currently pushing the envelope for ACL reconstruction in older yet active patients with complaints of instability.

To determine whether an ACL reconstruction is indicated in addition to HTO, the physician must consider the patient’s complaints at the time of initial presentation. If an older or less active patient is suffering from mechanical overload and pain, they will likely respond to the osteotomy alone. It is important to assess the entire clinical picture and differentiate pseudoinstability from true instability. If the patient continues to complain of instability after HTO, ACL reconstruction can be considered as a secondary procedure. However, ACL reconstruction alone in the face of malalignment is doomed for continuing symptoms of compartment overload and early failure of the ACL surgery.

Opening- or closing-wedge osteotomy can be performed in the varus knee with an alteration or decrease in slope. The senior authors utilize an OWO to correct varus, but can also decrease slope if required in the ACL-deficient knee. Finally, an anterior closing-wedge osteotomy to decrease the posterior tibial slope has been successfully suggested for the treatment of failed ACL reconstruction, with normal coronal alignment but posterior tibial slope >12–13° [18, 49]. Therefore, this option should be always considered in patients that sustain multiple ACL injuries, usually undergoing several unsuccessful ACL reconstructions that do not present evidence of technical errors, concomitant ligamentous laxities, or coronal malalignment. However, as knee instability represents the major complaint of these patients, the osteotomy should be combined with ACL reconstruction. Not much evidence exists to accurately assess these situations other than surgeon experience.

Since this is not a surgical technique-focused chapter but rather an evidence-based review, we will discuss preoperative planning and briefly the surgical technique principles involved. A more detailed description of the techniques can be found in other publications [6, 8, 49].

An accurate preoperative planning is mandatory in order to achieve the adequate correction, both on sagittal and coronal plane. Radiographic evaluation begins with assessment of the extent of knee arthrosis and lower extremity alignment with bilateral standard weight-bearing long-leg (hip to ankle) anteroposterior views, standard anteroposterior views in full extension, bilateral weight-bearing posteroanterior tunnel views in 30° of flexion, and lateral and Merchant patellar views (figure X-rays). MRI evaluation is helpful for preoperative planning, as it provides additional information that is often useful in determining soft tissue repair and reconstruction in addition to the osteotomy, such as chondral, meniscal, and soft tissue injury.

We prefer the medial opening-wedge osteotomy to the lateral closing-wedge osteotomy because, in our experience, precise correction is more likely and overcorrection is less likely. Although this approach increases the stability of a malaligned knee, it also avoids osteotomy of the proximal fibula, thereby avoiding potential instability through the tibiofibular joint and posterolateral corner structures and injury to the peroneal nerve [11, 30, 51]. Amendola and colleagues [2] have shown that by avoiding osteotomy of the proximal fibula, as with a lateral closing-wedge technique, the tibial slope will be forced to decrease because of hinging at the proximal tibiofibular joint.

The amount of axial correction is measured according to Dugdale et al. to avoid overcorrection [20]. The aim of correction may differ dependent of the underlying pathology. In ACL-deficient patients with varus malalignment and thrust, the correction may be aimed at a neutral leg alignment, whereas in patients with varus malalignment and unicompartmental medial OA, the aim is often to correct into valgus leg alignment to unload the damaged medial part of the joint. In the latter group of patients, we plan the osteotomy so that it will place the weight-bearing line—as measured from the center of the femoral head to the center of the tibiotalar joint to pass just lateral to the lateral tibial spine (or 62 % of the width of the tibial joint surface referenced from the medial side). In active patients who hope to return to a high activity level, the goal of correction may be a weight-bearing passing through the center of the knee joint at 50–55 %, even in the presence of medial cartilage damage, because an overcorrected leg would interfere negatively with their athletic abilities. In the setting of an arthritic knee with ACL insufficiency, the additional goal of the osteotomy is to achieve the desired posterior tibial slope in the sagittal plane to enhance stability of the knee [1, 17, 21, 44]. The surgeon must exercise caution in the setting of severe deformity, because the accuracy of correction may be more difficult to determine. Patients with osteoporosis present challenges in obtaining suitable fixation and can require prolonged periods for healing. Other considerations must be given to risk factors for failure, including smokers, prolonged dependency of corticosteroids, immunosuppressants, and chronic illness.

The senior authors do not perform any extensive articular cartilage resurfacing procedures such as autologous chondrocyte implantation (ACI) or meniscal transplantation at the time of this surgery. If they are required, surgery is usually staged; the osteotomy is performed first, followed by soft tissue reconstruction once the patient has recovered from the osteotomy.

As regards accuracy of correction, the wedge base length resulting from preoperative planning is intraoperatively measured and verified [8]. Intraoperative femorotibial alignment can also be verified by fluoroscopy, and an extramedullary alignment rod is used to ensure that the weight-bearing axis is passing through the center of the knee joint. Sabharwal and Zhao [45] have recently cautioned that for obese patients or those with substantial malalignment, supine fluoroscopy alignment measurements without loading of the knee joint do not reflect the axis as accurately as preoperative standing films. In such cases, we believe careful scrutinizing of the preoperative weight-bearing films and the intraoperative fluoroscopic images can still lead to favorable results.

The posterior tibial slope is also assessed intraoperatively and can be changed in opening-wedge valgization HTO by distracting the osteotomy more anteriorly or posteriorly if the patient has any symptomatic cruciate deficiency or excessive anteroposterior translation preoperatively. To allow for this correction in two planes, the hinge point must be cut and afterward compressed or separately fixed (Fig. 46.6 valgus-extension HTO). However, significant corrections of a highly pathological posterior slope cannot be obtained with this technique, and therefore if the major deformity to be corrected is in the sagittal plane, an anterior closing-wedge osteotomy should be preferred. When the desired opening has been achieved, the osteotomy is secured with a plate and, depended on personal preference the gap can be filled with bone graft.

In combined HTO and ACL reconstruction, an arthroscopy and preparation of the notch and femoral tunnel are performed prior to the osteotomy. The osteotomy is performed prior to drilling the tibial tunnel for ACL reconstruction to prevent the creation of a possible stress riser through the ACL tunnel. Arthroscopically assisted ACL reconstruction is done using standard technique with the following considerations. We drill the tibial tunnel anterior and superior to the osteotomy site. The ACL graft is passed through the tibial tunnel and out the femoral tunnel. The senior author’s preferences are to use extracortical button fixation. A tibial side interference screw can be placed for primary fixation proximal to the osteotomy site. Secondary fixation can be placed below the osteotomy site, if desired. Bone grafting of the osteotomy site is performed to accelerate bone healing.

Following surgery, the patient is allowed toe-touch weight-bearing with ROM performed within a 0–90° arc for 6 weeks. It is important to begin early postoperative range of motion to prevent stiffness in the knee joint. Radiographs are obtained at the 6-week postoperative appointment. If there is evidence of consolidation, the brace is discontinued and full weight-bearing is initiated with a strengthening program. At the 10-week postoperative appointment, radiography is repeated. If osseous consolidation has been achieved, sport-specific rehabilitation is initiated.

Knee arthroscopy and preparation of the notch are performed including the femoral tunnel using a rear entry guide or anteromedial portal. The tibial tunnel is performed after the osteotomy. An anterior longitudinal incision centered on the anterior tibial tubercle is utilized. The tibial tubercle is detached from the intended tibial osteotomy site as a 6-cm bone block. The closing-wedge osteotomy is performed according to the preoperative calculation. Under fluoroscopic control, one or two K-wires are inserted from anterior to posterior to mark the osteotomy site, starting about 3–4 cm distal to the joint line, parallel to the posterior tibial slope (PTS) (Fig. 46.7a). Keeping an intact posterior bony bridge is critical to protect the popliteal structures and limits the risk of secondary displacement or pseudarthrosis (Fig. 46.7b). The aim is to obtain a PTS of between 0 and 10° depending on the severity of the deformity and the knee motion. The anterior closing-wedge osteotomy is fixed with two staples or two “8” epiphysiodesis plates positioned medially and laterally with respect to the tibial tubercle. The tibial tubercle is repositioned by translating it distally with an amount equal to the thickness of the removed bony fragment to prevent postoperative change of patellar height, and it is fixed with two anteroposterior cortical screws: one above and the other below the osteotomy site (Fig. 46.7c). The tibia tunnel is then drilled in the standard fashion. Graft is passed from distal to proximal and secured with suture button fixation or screws (Fig. 46.8). Rehabilitation is similar to what is stated above.