A new and improved acetabular cup digital templating method and its clinical application

Abstract

Background and objective

Preoperative acetabular cup templating has an important auxiliary effect on hip surgery. The traditional acetabular cup templating method requires the measuring person to have some experience in total hip replacement (THA) surgery since the measurement results vary from person to person with differences between different measuring persons. To obtain stable templating results, we designed a new acetabular cup templating method and tested the inter-person measuring differences and measurement accuracy of this method. Meanwhile, the clinical application of this method was preliminarily explored.

Materials and methods

The pattern of this new method was manual labeling of imaging characteristic points and then programmed automatic measurements. The measurement process was performed entirely by orthopedic graduate students without any experience in hip replacement surgery. The inter-person measuring difference was evaluated by comparing the templating results of three measuring persons. The accuracy of the templating was evaluated by comparing the templating results with the actual size of the prosthesis in the surgery. The correlation between the position of the acetabular cup and the templating error was analyzed to explore the clinical significance of the templating results. This study was a retrospective study which included templating in a total of 406 cases for total hip replacement with cementless cup prosthesis. Digital measurements were performed using the Matlab software from MathWorks. The statistical comparison was performed using Kendall’s W test.

Results

The results of the three measuring persons were completely identical in 61.8% (251/406) of cases, and the variation in 38.2% (155/406) of cases did not exceed one size of the acetabular cup. The Kendall’s W coefficient was 0.977, and p < 0.01. The measurement accuracy is not as good as the traditional method in exactly accurate measurement and ±1 cup size, but it is similar to the traditional method in the ±2 cup sizes. The correlation between the templating error and the position evaluation of the implanted acetabular cups reveals: (1) larger the templating error, larger the proportion of the acetabular cups with poor position; (2) the proportion of acetabular cup with poor position slowly increased when the templating error was from 0 to 1 size, and the proportion rapidly increased when the templating error was from 1 to 2 size.

Conclusion

All the patients with clear teardrop bottom and lateral superior edge of acetabulum were able to use our method to predict the size of the acetabular cup. The method has the following advantages: (1) it does not require the measuring person to have any previous experience of the THA surgery, which reduces the labor cost of the templating; (2) the differences between the measuring persons is small, the measurement result can be repeated; (3) it can predict the probability of acetabular cup with poor positioning according to the templating error, and thereby reminding the surgeon to recheck and correct the position of the acetabular cup in time during the surgery.

Appendices

Appendix

Determination of measurement parameters

The significance of several variables was defined as shown in Table 5. (Fig. 7).

Table 5 Some variables used in this study and their significance

Fig. 7

Some variables and their significance

The above indicators for all cases in group 1 and group 2 were measured to find the median.

median_d_MD = 8.01 mm, median_d_LU = 8.17 mm, median_th_MD = 17.5°, median_cupsize = 52 mm. (Table 5).

Acetabular cup templating and the calculation of templating error

The program was executed to get the templating results and templating errors according to the following steps.

(1) For all the anteroposterior X-rays of the pelvis obtained from our hospital, a magnification of 8.09 pixels/mm was uniformly used;

(2) Let cup_size = 50 mm;

(3) Let d_MD = cup_size × median_d_MD/median_cupsize, d_LU = cup_size × median_d_LU/median_cupsize;

(4) In the dots_LU, a point closest to the d_LU from the boneedge_LU was found, named A;

(5) In the dots_MD, a point closest to the d_MD from the teardrop_BT was found, named B;

(6) The acetabular cup circle passed through A and B, so the acetabular cup center O was located on the mid-perpendicular line of AB; for the hemispherical acetabular cup, the ∠AOB was 180 − (18 + median_th_MD), for the sub-hemisphere/super hemisphere acetabular cup, if the center angle of the acetabular cup was angle_cup, then ∠AOB was angle_cup − (18 + median_th_MD); the diameter of the acetabular cup was calculated as d_cup_templating = L/sin(∠AOB/2), where L was the distance between AB. In addition, the coordinates of the center O of the acetabular cup was calculated and named as center_cup_templating; (Fig. 8).

(7) For the d_cup_templating measured in step (6), if the difference from the cup_size in step (2) was <0.3 mm, the process was continued to the next step; otherwise, let cup_size = d_cup_templating, and return to step (3), and continue the measurement;

(8) d_cup_templating was decimal, and the nearest double integer from the d_cup_templating was used as the final measurement result of acetabular cup size;

(9) By comparing the templating results of the acetabular cup and the results of the surgical record, the templating error of each case and the measurement accuracy of the entire sample were obtained.

Fig. 8

Determining the center and radius of the cup

Explanation of our templating method (Table 5)

To help understanding our templating method, we first simplify the templating conditions: all cups are hemispherical (the center angle of the cup is 180°), and the size of all the cups is about the same (close to the median 52 mm).

If it is known that a circle passes through two points A and B, it can be known that the center O of this circle is located on the perpendicular line of the line segment AB. If we also know ∠AOB, we can determine the position of the center O and the radius of this circle. In fact, there are two solutions for the circle center, which are located on both sides of the line segment, but considering the application scenario of the pelvic X-ray, the circle center O must be positioned below the line segment AB, so there is only one reasonable solution. To calculate the radius of the cup circle, we need to first determine A, B, and ∠AOB, and then determine O, and then the length of the AO is the radius of the cup.

On the AP X-ray of the pelvis, A is the lateral superior intersection of the cup and the inner wall of the acetabulum. The distance between it and the lateral edge of the acetabulum is d_LU. By measuring the d_LU on postsurgery X-ray of all 406 cases, the median of d_LU is obtained. When templating a new preoperative AP X-ray of the pelvis, as long as we know the coordinates of the lateral edge of the acetabulum and the contour of the nearby acetabular edge, combining with the value of median_d_LU, the ideal position of A can be estimated (A is located on this contour and its distance from the lateral edge of the acetabulum is median_d_LU). In a similar way, the ideal position of B, the medial inferior intersection of the cup and the inner wall of the acetabulum, can also be obtained.

By measuring the th_MD on postsurgery X-ray of all 406 cases, the median of this angle can be obtained (median_d_MD); the lateral uncoverage should not exceed 20%. It can be known that the average coverage ratio is 10%, then the ideal angle on the lateral side should be 180° × 10% = 18°, so the ideal ∠AOB should be equal to 180° − (18 + median_d_MD).

In this way, A, B and ∠AOB are obtained, and then the position and radius of the cup can be calculated.

The above is a simplified templating process. In clinical application, some cups are sub-hemisphere or super-hemisphere, we need to adjust the center angle of the cup, which will affect the value of ∠AOB (See templating step 6 in "Appendix"). In addition, median_d_LU and median_d_MD is suitable in the cases when the cup size is equal to median_cupsize. If the measured cup size is not equal to median_cupsize, we need to scale median_d_LU and median_d_MD (See templating step 3). Because we do not know the templating results in advance, the ideal value of cup_size/median_cupsize is also unknown, therefore, we let the computer try different cup_sizes repeatedly until the templating result becomes satisfactory (see templating step 7).