Accurately assessing acetabular defects and making surgical plans accordingly are critical to ensure successful surgeries(5). However, current acetabular defect classifications have kinds of disadvantages(8, 10, 11, 24–28). Campbell(10) assessed the reliability of three major acetabular classifications described by Paprosky(6), the AAOS(7) and Gross(9). The innovator group reached only moderate range of intraobserver agreement and for the non-innovator group, the intraobserver and interobserver agreement was even worse for all 3 classifications assessed. Gozzard(25) reported Paprosky classification system achieved moderate to good levels of agreement and bone stock loss classification systems were inconsistent and unreliable, which prevented realistic comparison of results within or between centers. Johanson(11) reviewed six acetabular defect classification systems and concluded that only one of them demonstrated required reliability and validity for a standardised grading system. Most current classifications only roughly guide surgical plans which need to be decided by specific perioperative situation.(6, 11, 13, 14) Campbell(10) claimed that the Paprosky system should be “considered only as a general guide” for its poor reliability. As a result, the degree of bone defects found during surgery may exceed surgeons’ expectations. However, adjusting surgical plan intra-operatively is difficult and insufficient preparation may lead to surgical failure and worse post-operative outcomes. In addition, current classifications have greater significance in evaluating relatively simple defects than complex cases(14). Ghanem(13) reported 16.37% of cases had further defects found intraoperatively than the preoperative plan and he suggested a practical, reproducible and valid classification system for preoperative planning.
One reason for this is that previous classifications were mostly based on traditional radiological techniques. It is difficult to realize 3D structures of bone defects through X- rays or CT model on 2D computer screens and information is always incomplete(15–17). With 3D printing technology(18–22), 3D reconstructed RP displays 3D anatomy structure more intuitively and provides key information difficult to obtain from 2D images. Thus, RP aids surgeons to understand bone defects more intuitively and formulate surgical plans more comprehensively, which reduces errors and improves postoperative outcomes compared to x-rays or CT model(22).
For this reason, we proposed our 3D acetabulum defect classification system aided with RP. Since Type I in this classification was little defected and it was easy to distinguish from X-rays or CT, it was unnecessary to print RP models for such patients or take Type I cases into reliability and validity test.
This study showed both classification results and surgical plans were of good inter and intraobserver reliability and high validity compared to surgical records. It suggested our 3D classification helped classifying acetabular defects preoperatively and guiding reliable surgical plans. There might be several reasons for this. First, comprehensive information was accessible. In relative complex cases, RP presented bone defects and the rest bone mass through visible 3D models. Hence, demand on radiograph reading was less and no information was lost for evaluation. It became easier for surgeons to realise the real 3D structure of defected acetabulum. As a result, three surgeons made classifications and surgical plans of high intraobserver reliability. Second, anatomy structures displayed by RP was so intuitive that a surgeon gave a closing classification at the second time after the first impression, which explained high interobserver reliability. Third, evaluations were based on objective RP model, which could be utilised for surgery simulation to test plans’ feasibility and reflect intraoperative situations preoperatively. Thus, classification results and surgical plans were of high validity. It was consistent with previous reports. Preoperative planning with RP before hip revision improved clinical results(18–22). Involving RP improved diagnosis accuracy in discovering fractures in complex anatomical areas, such as obscure pelvis fracture(29, 30).
In this classification, the leading factor to be considered is how to obtain the primary stability of a cementless cup, including rotational and vertical stability. Primary stability can be obtained either by effective three-point fixation distributed over 180 degrees or by enough screws fitting into wall.
We chose the anterosuperior acetabulum, the ischial ramus and the pubic ramus for a three-point fixation because other structures around the acetabulum are relatively thin and often destroyed in severely defected cases. Despite some bone mass remains, this kind of bone is of high density due to osteosclerosis. It is brittle and unable to provide effective holding. The three areas mentioned above have larger interface and thicker bone stock. The remaining bone will still be sufficient for holding even if osteolysis occurs around. Although the ischial ramus and anterosuperior acetabulum have a relatively large bone stock and some surgeons even believe the cup can be held with only these two points, it should be avoided as much as possible to our opinion. This is exactly the difference between type III and type II. In some type III cases, defects in the anterosuperior acetabulum or the ischial ramus can still be reconstructed with a buttress plate or augments and a three-point fixation can be obtained combining other two places. If more than one place needs reconstruction, the above method might not provide sufficient holding and stable three-point fixation is important(13).
We classified cases with internal acetabular wall defected but an intact acetabular ring (Paprosky type 2C) into type II because when the remaining bone is enough to provide a three-point fixation, it is still feasible to hold the cup with augments or screws.
If type II has defects in the stress-bearing posterosuperior acetabulum, we believe the direct supporting between bone and prosthesis is the basic principle for achieving reliable long-term stability through bone remodelling. When structural or compression bone grafting is used, the progress of bone remodelling is so long that weight-bearing on the interface between the prosthesis and graft is inevitable. As a result, the prosthesis gets loosening quickly owing to micromotion. Our cases found that when there was no direct contact between the prosthesis and host bone but through allografts, although most allografts were well reconstructed, fatigue fracture occurred in the connection part to the cage due to micromotion (Figure 10). It is possible that the remoulding progress was so long that the grafted bone could not provide sufficient support to the prosthesis in the weight-bearing area although the grafted bone was well remodelded finally.
With cavity or uncontained defects in the weightbearing posterosuperior acetabulum as type II, augments are needed. When bone defects are not severe and the host bone thickness is still sufficient, conventional commercial augments can repair the defects and cup can be fixed by remaining acetabulum wall or screws. When uncontained defects or severe defects exist, stable fixation cannot be achieved. Despite some bone mass in adjacent ilium in some cases, it is difficult to directly fix an augment on to it. Customed augment fixed wings or a buttress plate is required.
We noticed few patients with defects only in the anterosuperior acetabulum in clinical practice. It is possibly because rare force passes the anterosuperior. Anterosuperior defects always occur together with posterosuperior defects, which result in defects in the superior acetabulum (type III). When severe osteolysis only occurs in either the superior acetabulum or the ischial ramus, a buttress plate or a customed augment with a wing can be used to reconstruct defected places to hold the cup. When more than two of these places are defected, the primary vertical and rotational stabilities cannot be obtained using conventional methods. Cup-cage or customised prostheses is recommended. At present, most commercial Cup-Cage is used when there are no serious bone defects at the bottom acetabulum. Cup-Cage can be fixed by an obturator hock into the obturator foramen or a hook bound to the upper edge of the obturator. Customised cages can be used for defects in many areas when rotational stability is difficult to obtain.
There are some disadvantages in this study. First, the original size of each patient’s pelvis and remaining bone is different and it is difficult to classify quantitatively. However, only with this qualitative 3D classification, professional surgeons qualified for joint revision can already distinguish the extent of bone defect and design feasible surgical plans, which already accomplishes the goal of surgical planning. Second, the accuracy of showing effective bone mass by RP needs further improvements(18). However, current accuracy already meets the needs for most clinical applications. Our recommended surgical strategies were feasible, but not the only method. Other experienced surgeons might have their own understandings and corresponding reconstruction methods.