12 Radiological examination of bone healing after open-wedge tibial osteotomy

1 Introduction

The general principles of fracture consolidation apply equally to the healing process of osteotomies. Osteotomies can be regarded as iatrogenic and targeted fractures, which are placed under optimal conditions and circumstances in order to reduce and stabilize the resultant bone fragments in the corrected position. The type of the deformity and its localization, the exact amount of correction, and the direction of correction must be considered when planning an osteotomy. Stabilization is performed according to the principles of internal fracture fixation.

Both adequate stability and good vascularity of the bone and soft tissues are premise for good bone healing and, consequently, for the good long-term results of open-wedge valgization osteotomy of the proximal tibia.

In addition to clinical aspects, radiological parameters and histological examination provide data to quantify bone healing. Regarding bone healing after open-wedge tibial osteotomy, the question often arises as to whether the interposition of solid material or osteoconductive/osteoinductive substances into the osteotomy gap is beneficial. The author has performed over 500 open-wedge osteotomies with the TomoFix implant without filling the osteotomy gap.

Bone healing

The pattern of consolidation after bone injury depends on the size of the defect and biomechanical stability of the internal fixation. There is a basic differentiation between primary bone healing (contact healing) and secondary bone healing (gap healing). Primary bone bridging under stable conditions is possible to a threshold of 0.5 mm [1].

In primary healing (contact healing) every osteon passes through a regeneration cycle divided into three phases: activation, resorption, and formation (ARF sequence) [2, 3]. A basic cellular unit consisting of osteoclasts, an axial blood vessel, and osteoblasts performs the remodeling sequence. The osteoclasts extend the resorption canal in an axial direction, whereas the osteoblasts are responsible for centripetal apposition of new bone lamellae that gradually narrow the resorption canal concentrically until the definitive dimension of the Haversian canal is reached. The apposition rate is about 1 μm per day. The lamellar osteoid matrix mineralizes after about 8–10 days. This process of restructuring is referred to as remodeling of the Haversian system [1].

In secondary healing or gap healing bone regeneration occurs directly without preceding osteoclastic resorption or intermediate cartilaginous phases. It is a two-stage consolidation process. During the first phase a trabecular scaffold of fibrous bone forms within 1–2 weeks and its meshes are later filled out with lamellar bone [1]. This structure is replaced in subsequent weeks by osteons running perpendicular to the fracture gap. Secondary bone healing takes place where micromotion between the fragments occurs. In plate osteosynthesis, callus always forms at the contralateral cortex. In internal and external fracture fixation or metadiaphyseal osteotomy primary and secondary instabilities at the contralateral cortex cause biomechanical stress on the fixation system.

3 Revascularization and bone remodeling after fractures and osteotomies

Both biomechanical stability and adequate blood supply are vital to fracture healing [4–11]. The earlier the traumatic and iatrogenic vascular damage, eg, due to heat necrosis during sawing, is resolved by restoration of cortical blood supply, the more rapidly the bone heals [5, 6, 9]. The revascularization process begins within the compacta and is attended by intracortical remodeling in the zone between the well vascularized and the poorly vascularized bone. Necrotic material is resorbed and replaced by regenerated bone [12]. The first basic cellular unit consists of osteoclasts and osteoblasts with a central blood vessel. These reach the medullary cavity after 4 weeks. Bone remodeling is a relatively slow process [13–15], whereby the process depends on cortical thickness. Experimental studies have shown that remodeling in the rabbit with its relatively thin cortex takes 4 weeks, but will take 12 weeks in specimens with thicker cortices, eg, sheep or dogs. Duration of more than 12 weeks is assumed in human individuals [5, 6, 13].

4 Open-wedge tibial osteotomy with autogenous and heterogeneous interposition material

Open-wedge osteotomy of the proximal tibia creates a highly unstable situation, but it also allows for correction and stabilization in various degrees of freedom ( Fig 12-1 ). Whether open-wedge valgization osteotomy of the tibia should be performed with or without interposition material is currently under discussion. Several reports in literature describe the interposition of autogenous materials (generally one or more bicortical iliac crest grafts) or insertion of homogenous and heterogeneous bone substitutes in medial open-wedge tibial osteotomy [16–25]. As for synthetic materials, hydroxylapatite (HA) is generally preferred and can be inserted into the osteotomy gap in the form of compressed wedges to supplement plate fixation. Koshino et al [21] investigated these substances histologically and found a bone integration rate of 72% in the HA pores. Plain x-rays did not show any alteration of radiological density of the osteotomy gap when filled with the HA wedge. The planned mechanical axial alignment of 3-6° valgus was achieved in 75% of patients in this study. The author reports an average axial correction of 10.3°. No loss of correction in the subsequent course, however, two cases of delayed healing and one case of pseudarthrosis were recorded.

**Fig 12-1** Correction in different directions can be achieved by open-wedge tibial osteotomy. Combined corrections can be useful in cases of complex deformities.

Hernigou and Ma [19] investigated 245 patients after medial open-wedge HTO stabilized by application of conventional plates and insertion of acrylic bone cement into the osteotomy gap. They present this procedure as an efficient method of preventing secondary loss of correction but do not specify the contribution of the cement to stability.

5 Open-wedge tibial osteotomy without interposition material

The principles of angular stable internal fixation with the TomoFix implant have been presented in detail in chapter 7 “Principles of angular stable fixators” and chapter 9 “High-tibial open-wedge valgization osteotomy with plate fixator”. Currently, surgeons who apply this implant to stabilize the tibia after open-wedge osteotomy are not definitively agreed on the maximum gap width beyond which autogeneous or heterogeneous graft is indicated. Galla and Lobenhoffer (see chapter 9 “High-tibial open-wedge valgization osteotomy with plate fixator”) recommend filling the gap from a vertical width of 13 mm and more, but in the author’s own procedures and those of other working groups the gap is either not filled or only filled from a vertical width of 20 mm or more [18, 26].

With regard to consolidation after open-wedge osteotomy without interposition material, two basic questions remain:

How does new bone regenerate in the osteotomy gap?
Does the regenerated bone correspond to normal bone in terms of biomechanical stability and stability under load?

5.1 Evaluation of x-rays

Preoperative imaging for correction osteotomies are described in chapter 2 “Clinical and radiological evaluation”. The postoperative evaluation requires an AP radiographic view of the knee joint, lateral view in 90° flexion, and a full-length weight-bearing view immediately postoperatively and after 6, 12, 24, and 52 weeks. It is important to ensure neutral rotation and complete extension of the knee joint to obtain images suitable for evaluation.

Bone healing

The author’s experience indicates that bone healing in the osteotomy gap progresses from lateral to medial [27]. A lateral bone bridge is left intact intraoperatively, ie, the so-called hinge, which is defined at about 10% of the bone width (see Fig 12-2a ). This hinge is not cut during osteotomy and yields to plastic deformation when the osteotomy gap is opened (see chapter 9 “High-tibial open-wedge valgization osteotomy with plate fixator”). The author finds a distance of 5-10 mm to the lateral cortex to be optimal.

An early sign of bone regeneration is an increase in bone density at the osteotomy surfaces. Band-shaped zones of new bone form at the osteotomy surfaces as part of the healing process ( Fig 12-2 ). In this way, the osteotomy gap is successively consolidated from lateral to medial. Awfter 6 weeks bone contact is seen along almost one-third of the osteotomy surface at the interface between the tibial head and the tibial shaft. The author observed very rapid bone consolidation at the ascending anterior osteotomy (often already after 3 weeks). This area acts as a stabilizing column during the healing process. The anterior osteotomy accounts for about 5% of the total bone contact surface.

Table 12-1 and Table 12-2 show the series of patients and the progress of bone consolidation in the osteotomy gap for our study of 53 patients.

Initial bone resorption is seen in 5 5 % of all patients 3-4 weeks postoperatively (see Fig 12-3, Fig 12-7 ). This leads to slight temporary instability with increased load on the implant. This phenomenon was seen more frequently in smokers and after secondary fracture of the lateral hinge. In 10% of all cases the latter caused displacement of the lateral cortex by more than 5 mm. This occurred in the first phase of the learning curve immediately after implant development ( Fig 12-4 ). It was noticeable that the fissures extending distally were much more unstable than transverse fissures extending laterally or slightly ascending fissures. In severe deformity requiring more extensive correction with a larger osteotomy gap, a lateral hinge fracture is unavoidable, especially if full extension needs to be reached through the osteotomy ( Fig 12-6 ). Nevertheless, temporary insertion of a compression screw into combination hole 1 will resolve this problem. The screw pretensions the plate and leads to secondary reduction and compression of the lateral hinge (see chapter 9 “High-tibial open-wedge valgization osteotomy with plate fixator”). The formation of callus at the posterolateral tibia during the course of consolidation is considered as a sign of temporary tilting instability ( Fig 12-3, Fig 12-7 ), which is triggered by increasing pressure in flexion and which must be neutralized by the implant. However, there is no correlation between the extent of the cortical fracture and the intensity of callus formation.

**Table 12-1** Basic data.
	Average age in years	Number of male patients	Average width of the osteotomy gap in mm
Patients (n = 53)	50 (18-71)	39	10 (5-20)

**Table 12-2** Progress of bone consolidation in the osteotomy gap at the postoperative AP radiological follow-ups.
Postoperative radiological follow-uo (Ap view)	Total bone-contact surface in%
Day 1	23± 15(0-50)
3 weeks	25 ± 15 (10-60)
6 weeks	32 ± 20 (0-60)
3 months	57 ± 14 (30-80)
6 months	73 ± 14 (50-100)
9 months	80 ± 10 (70-90)
12 months	88 ± 10 (80-100)
At implant removal	90 ± 10 (80-100)
After implant removal	94 ± 6 (80-100)