One treatment modality for medial osteoarthritis of the knee is open-wedge high tibial osteotomy (HTO). The current most common way of performing HTO is to use fixation plates with locking bolts implanted in the patient's tibia, to allow increased stability \& joint movement and the ability to be weight bearing on the affected leg sooner . However, despite advances in high tibial osteotomy technology, it can still take most patients about six months to fully recover from the procedure. Sedentary activities are usually picked up about a month after surgery, before physical work is introduced 3 to 4 months after surgery, followed by sports which can be reintroduced approximately 6-12 months after surgery. One reservation for allowing patients to be fully weight bearing early on is the perceived risk of loss of correction of the angle, although in practice this effect appears to be a rare occurrence . For example, in a series of patients Takeuchi and colleagues showed that full weight-bearing is possible two weeks after surgery without implications for the correction made to the joint angle .

It is imperative that the open wedge is healed and repopulated by new bone, to strengthen the tibia and allow full recovery following HTO. Regeneration will take place naturally, although some surgeons apply aids to promote bone healing, such as the ChronOS β-tricalcium phosphate wedge. However, research into this aspect of filling the wedge has shown that there is no advantage to using the filler - both in terms of stability and bone healing time of the wedge . Although fixed systems like Tomofix facilitate early weight-bearing of the affected joint, the complete closure and consolidation of the wedge takes considerably longer. In one study, 23 /27 (85%) osteotomies were fully healed twelve months after procedure . Histologically, there is variability in the degree of healing and indeed maturation of bone regeneration achieved in the tibial wedge. With current fixed plate devices, even two years post-procedure, a minor subset of patients will not have significant signs of regeneration in the surgical site. In the majority of patients, partial bone regeneration with fibrocartilaginous tissue takes place and in some patients by new regenerated fibrocartilage or hyaline-like cartilage will appear .

HTO could be improved upon to allow an earlier return to activities by patients and to achieve better rates of healing - both in terms of time and maturity of the regenerated bone. Ellipse Technologies have devised a new nail system for high tibial osteotomy that differs from the established 'locked' plate and nail systems. Their system functions in a more gradual fashion. The wedge is created in the tibia as per normal but then the Ellipse nail is inserted and per day the nail is extended in small measures (appr 1.5 mm per day) until the desired correction has been achieved. Currently the new Ellipse system is being trialed in various countries in Europe, including the Netherlands and the UK. The main Ellipse trial is seeking to establish whether the corrective power of this medical device is more accurate than the plate/nail systems in use at present. The degree of bone regeneration in the open wedge is reported on, but not in detail - e.g. by one reporting radiologist - and by using standard X-ray imaging which is primarily used for angle measurements.

This present study seeks to explore in more detail how the Tomofix and Ellipse high-tibial osteotomy systems function in relation to bone regeneration in the open-wedge created as part of the procedure by utilising CT imaging techniques rather than standard clinical X-ray imaging. The primary aim of osteotomy is realignment of the joint and thereby preserves the joint so that the patient can continue to enjoy the active life style, sports and recreational activities. After an osteotomy procedure, whenever a patient asks for advice to the clinician when they can start participating in unrestricted sports activities, clinicians often find it very difficult to answer this question. This is primarily because it is often very difficult to quantify the bone healing process precisely on radiographs. By quantifying the amount of bone healing on CT scan which is more clear and accurate, the clinician can confidently advise the patient regarding the participation in various sports activities without ambiguity.

Further, by comparing the bone healing process between Tomofix plate and Ellipse nail, investigators can make more informed judgement regarding their performance. This research will also enhance the choice of implant selection in osteotomy patients based on the duration of bone healing.