Pain can be a problem and often leads to unnecessary revision.
It is most commonly encountered over the proximal tibia and is anteromedial in distribution. This type of pain is not unusual in the first six months and usually settles spontaneously. Review of our patients showed that the incidence has decreased with time and is now 2% at one year follow up (Gulati 2013). In other series, it has been higher, particularly with surgical inexperience (Gulati 2013). Other sites of pain are much less common. The NZJR reports pain as a cause of revision in 38% of UKA revision and data from the NJR suggests 23% of UKA revisions are for pain.
There are numerous proposed causes and many may be multifactorial. Unexplained pain is the most common presentation but there is increasing evidence (Pegg et al, 2013; Small et al, 2010) that inappropriate indications or bone overload are the most common causes. Impingement, soft tissue irritation, cementing errors, pes anserinus bursitis or neuroma have all been implicated .
Partial thickness cartilage loss (PTCL)
It is generally thought that UKA is best used in young patients with early arthritis. We strongly disagree with this and recommend that the OUKA is only offered to patients with bone-on-bone arthritis. Cadaveric studies have shown that asymptomatic PTCL is common (Emery & Meachim, 1973). So, if a patient has pain and PTCL, the PTCL is not necessarily the cause of pain. In a study (Pandit et al, 2011) comparing the outcome of patients with PTCL and matched patients with bone exposed (BE) or bone loss (BL), it was found those with PTCL had worse outcome scores and greater variability than BE and BL (OKS 36 (SD 10) v 43 (SD 4) & 43 (SD 5) respectively). Furthermore, 21% of the PTCL group were worse or had no substantial improvement (ΔOKS<6) after the surgery, whereas all patients in the BE and BL groups reported substantial improvement (Fig. 11.13). In the study, there were four complications, all of which were pain related and all occurred in the PTCL group (Pandit et al, 2011). Although some patients with PTCL do well with OUKA, a sizeable proportion do not. Until it can be predicted which will do well, it is sensible to avoid doing UKA in patients with PTCL. In the future, it may be possible to predict which will do well with a bone scan or MRI, but as yet this has not been shown to be possible. It is therefore important to be able to distinguish between those with PTCL and those with bone-on-bone. We do this with a series of radiographs including standing AP, varus stress or Rosenberg. If there is preserved joint space on these views, we would then do an arthroscopy and only proceed to OUKA if exposed bone is seen on both sides of the joint. If there is not bone-on-bone, we would treat the patients conservatively. The pain either tends to improve or, if the arthritis worsens, a UKA can be performed.
Figure 11.13 Comparison of post-operative OKS in matched patients with Partial Thickness Cartilage Loss (PTCL), Bone Exposed (BE) and Bone Loss (BL).
Niinnemaki et al. (2011) in a series of 113 OUKA from a Finnish centre found that the re-operation rate was directly related to medial joint space on standing AP radiographs before surgery. The greater the joint space prior to surgery, the higher the re-operation rate after surgery. If it was ≤2 mm, then the re-operation rate was six times lower than when the medial joint space was more than 2 mm. If the joint space was normal, the re-operation rate was about 70% (Fig. 11.14).
Figure 11.14 Re-operation rate according to pre-operative medial joint space width.
Bone overload
Both cadaveric studies (Small et al, 2010) and finite element analysis (FEA) studies (Pegg et al, 2013) have shown that, following UKA, the tibial strain increases anteromedially below the tibial component (Fig. 11.15). This may explain why anteromedial pain occurs postoperatively and why the pain settles as remodelling occurs and the strain returns to normal. The FEA demonstrates a 60% increase in strain with a perfect tibial resection. There is a further increase in strain with a deep vertical cut, a medial vertical cut and a deep tibial resection (Pegg et al, 2015). Clearly surgeons should take all measures to avoid these errors. The Microplasty instrumentation was designed to help address these issues.
Figure 11.15 (a) Cadaveric and (b) FEA studies showing increased strain anteromedially below the tibial component where pain often occurs (Pegg et al, 2013; Small et al, 2010).
Cementing errors
Cementing errors are a common cause of pain. They may result in loose bodies (Fig. 11.16), soft tissue irritation from medial overhang, a tight flexion gap, and impingement.
Figure 11.16 Radiograph and retrieved specimen from a patient with pain from cementing errors. Too much cement was used resulting in the tibial component not fully seating, a loose body, and bearing impingement.
Component overhang
We performed a study to assess the impact of tibial component overhang (Chau et al, 2009). We found that medial overhang of more than 3 mm was associated with pain and poor function that tended to get worse with time. Presumably this was due to irritation of medial soft tissues (Fig. 11.17). The tibial component increases in size parametrically by 2 mm so overhang of 2 mm or more can be avoided by selecting the appropriate component size or redoing the vertical cut further laterally.
Anteromedial femoral component overhang may also cause pain and should be avoided.
Figure 11.17 The relationship between overhang and change in OKS. Major overhang is more than 3 mm and minor overhang less than 3 mm.
Anterior impingement
Anterior impingement of the bearing against the femur in extension can occur if there is inadequate removal of anterior femoral bone. This can cause pain in extension and tends to develop with time as the fixed flexion contracture corrects.
