The purpose of this study is to assess whether there is a difference between the TomoFix and Ellipse Intramedullary HTO Nail System in terms of bone regeneration in the tibial wedge by performing a CT-scan at 3 and 6-months postoperatively.

Malalignment of the knee joint causes arthritis in later life. Currently there are three surgical ways to treat knee arthritis: total knee replacement, partial knee replacement, and high tibial osteotomy (HTO). The former two very much focus on treating the effect of malalignment, i.e. removing the arthritic joint. However, HTO addresses the cause of the arthritis, namely the joint malalignment. This preserves the patient's joint and it means that a patient will start using the unaffected part of the knee joint more.

HTO involves cutting a wedge out the tibia to correct the alignment of that bone with the knee joint, to redistribute load from the affected medial part to lateral part. To keep the tibia in the new position, a medical nail device is attached to keep it in place and allow new bone to regenerate within the wedge. Currently, the market leader for HTO is the TomoFix (by DePuySynthes company) plate and nail device. As the name suggests, TomoFix is fixed at surgery and therefore the change in bone angle cannot be changed afterwards. It does mean that patients can be weight-bearing on the affected leg soon after the HTO procedure. A new CE-marked device is being tested in an interventional trial; it is produced by Ellipse Technologies. This device is an extendable nail and inserted intramedullary; following surgery the nail is tend slowly extended over a period of time until the bone correction is satisfactory.

The CISKO imaging study will assess whether there is a difference between the TomoFix and Ellipse system in terms of bone regeneration in the tibial wedge by performing a CT-scan at 3 and 6 months post-operatively. This will be quantified by two independent radiological reports. A secondary objective is to investigate patient satisfaction and also patient pain levels at these time intervals. The degree of bone healing is usually the main factor holding clinicians back when it comes to advising patients on what activities they can return to post-operatively. A difference in bone healing between the two systems may impact on the advice clinicians can give patients regarding recommencing more intense activities such as recreational sports, which ultimately could positively impact patients' health and well-being.

Inclusion Criteria:

1. Indicated treatment with medial open wedge proximal tibial osteotomy, either with Tomofix device or Ellipse device
2. Provision of written informed consent
3. Males
4. Mental capacity

Exclusion Criteria:

1. Under age (< 18 years)
2. Patients lacking mental capacity.
3. Females
4. Current use of nicotine products.
5. Patients who cannot understand English and therefore cannot be consented.