# Personalised High Tibal Osteotomy Surgery Is Accurate: An Assessment Using 3D Distance Mapping
## 1 Introduction  
Osteoarthritis (OA) of the knee carries a huge personal [1] and societal burden [2, 3]. The global prevalence of knee OA is high, being highest in Asia (19.2%), followed by North America (15.8%) and Europe (13.4%) [4]. Incidence of knee OA is strongly associated with increasing age [5]. As the global population ages, the prevalence of the disease is expected to increase [6] and to have a significant impact on healthcare systems for the coming decades [7]. In addition, previous predictions of OA prevalence have underestimated actual increase in the burden of this disease [8], and the increase in prevalence is greater than explained by ageing and increased BMI [9]. Knee OA is associated also with increasedcardiovascular disease and mental health conditions [10], as well as greater than a 15% increase in premature all-cause mortality [11]. Joint replacement is considered as the gold standard in treating knee OA. The demand for this treatment is growing and projected to at least double from 2015 levels by 2030 [12], with studies even predicting far higher demand [13]. However, joint replacement is only applicable for end-stage disease [14], and the majority of OA patients fall into the treatment gap, possibly for decades [15]. Nonsteroidal anti-inflammatory drugs and opioids are the most prescribed pain management medications for OA [16]; however, this form of treatment is associated with poor clinical outcomes, increased mortality, and healthcare resource utilisation [17]. It is worrying that the prevalence of opioid prescription has increased, as a 44% prevalence of patients prescribed opioids while waiting for knee replacement has been recently reported [18]. The majority of knee OA begins in the medial compartment [19] and it is often suitable for treatment with unicompartmental knee replacement (UKR) or high tibial osteotomy (HTO), which have broadly similar indications [20]; however, it is recommended that HTO is used at earlier stages of OA [21]. The consequences of medial knee OA is that the affected knee bows out laterally, which is termed varus knee alignment. HTO surgery aims to correct this alignment closer to neutral. The alignment of the knee is clinically measured by the angle between the mechanical axis of the femur and that of the tibia in the frontal plane, termed the hip-knee-ankle (HKA) angle. The convention used in this work is that HKA > 180\({}^{\circ}\) is varus alignment. Patient outcomes are generally reported to be similar between UKR and HTO, with larger range of motion and more activity observed after HTO [21; 22; 23]. Recently a number of studies have shown a relatively high early failure rate for UKR. Based on data from the United Kingdom National Joint Registry, UKR revision rates are 2.5 times higher than those from total knee replacement for UKR surgeries performed by the highest quartile volume surgeons, and even 3.9 times higher for those performed by the lowest quartile [24]. This concurs with a previous report based on United States data [25], and also with another report based on the Italian regional registry data [26]. The 2021 United Kingdom National Joint Registry report shows that UKR revision rates are approximately 15% for males and 16% for females at ten years after surgery, rising to 22% for males and 24% for females at 15 years [27]. There is a clear need for effective joint-preserving treatments that can be utilised at a relatively early stage of knee OA. HTO has a well-established clinical track record and is suitable for relatively young patients. However, HTO surgery is perceived as technically challenging, and surgeons are concerned about achieving their planned correction [28]. Achieving the planned correction is difficult, and the clinical outcome is apparently highly dependent upon this [29; 30]. This perception may be the major reason for a decline in HTO surgery in the last two decades [20].