Phi Angle and Antero-medial Pain in Total Ankle Replacement Follow-up

Background:

In case of total ankle prosthesis surgery, the correct positioning of the talar component is one of the most challenging tasks. When performing total ankle replacement, the primary goal for long-term implant survival is to restore a physiological alignment of the prosthesis in the lateral and frontal planes. In fact, an incorrect alignment can alter joint mechanics and joint reactive forces that can lead to implant failure (Saltzman, 2004).

Regarding the rotation in the dorsoplantar plane, the prosthesis is aligned taking as reference the axis of the second metatarsal bone, with the foot in neutral position (Roukis, 2015). In this regard, there are no specific mechanical-articular considerations as for the lateral and frontal planes. However, it is expected that misalignments of the prosthesis in the dorsoplantar plane may involve the presence of antero-medial pain in the ankle in the long term (at least 1 year after surgery).

At present, no previous studies have verified whether there is a relation between the rotational alignment of the talar component in the dorsoplantar plane and the frequency of antero-medial ankle pain, at least 1 year after surgery. This evaluation can provide useful information for a more advantageous placement of the implant during the intervention.

Aim of the Study:

The aim of the study is to evaluate the relation between the rotational alignment of the talar component (phi angle) and the antero-medial ankle pain, in patients underwent total ankle replacement, at least one year after surgery.

Moreover, the study aims to define a range of the phi angle associated with a lower frequency of the antero-medial pain.

Methods:

A sample of 100 subjects in the follow-up period after total ankle replacement (minimum 12 months after surgery, maximum 36 months) will be enrolled. Age range will be 18 to 60 years. Each subject will be evaluated once, in occasion of the routine follow-up examination. The subjects will be enrolled consecutively and differentiated according to the time passed since the intervention (at least 12 months), with incremental windows of 6 months, up to a maximum of 36 months. This will lead to define 4 time windows (12-18 months, 18-24, 24-30, 30-36). A number of 25 subjects will be enrolled for each time window. Once a time window will be completed, only subjects belonging to the remaining not completed windows will be enrolled.

Outcomes:

The phi angle will be measured according to that described by Manzi et al., 2017. Specifically, the phi angle will be obtained by the orthopaedist on the standard dorsoplantar radiograph of the foot, which is routinely performed for the follow-up examination. The phi angle is the relative angle between the alignment of the talar component of the implant, and the axis passing through the second metatarsal bone, in the dorsoplantar plane. The phi angle is defined as negative when the talar component is aligned in an abducted position to the second metatarsal axis; similarly, the angle is defined as positive when the talar component is aligned in an adducted position.

The rate of postoperative pain will be assessed during the follow-up examination by a recently distributed pain monitoring web-based app: 'NavigatePain' (NavigatePain, Aglance Solutions, Denmark). The app is used in a simple and intuitive manner by exploiting tablet device (Boudreau, 2016; Matthews, 2018). It is used only for data collection, without therapeutic or diagnostic purposes. In few minutes, it allows the patient to draw the pain areas in the frontal and lateral planes, automatically providing an index that quantifies the percentage of anterior and medial areas affected by pain (with respect to the whole body area in the considered plane).

The antero-medial ankle pain will be obtain as the sum of the anterior and medial pain indexes provided by the app.

The strength of the relation between the phi angle and the antero-medial ankle pain will be evaluated according to Pearson correlation coefficient, or Spearman rank correlation in case of not normal distribution. Statistical significance of the coefficients will be tested according to two-tailed t-test or permutation distribution test assessing Pearson and Spearman coefficients, respectively. Linear regression analysis will be performed as well.

Correlation coefficient and linear regression will be evaluated in each time window. The results will be compared among the consecutive windows in order to assess the potential dependence on the follow-up time.

The sample size of 25 subjects per time window has been verified to guarantee identifying as significantly different from zero a correlation value larger than 0.7 (i.e. strong correlation), by t-test at 0.05 level alpha and power 95% (G*Power software, Universitat Dusseldorf, Germany).

The distribution of the pain rate will be evaluated in function of the phi angle. Potential threshold values of phi associated with lower pain rates will be identified, in order to define a range of phi associated with a lower frequency of the antero-medial pain.