Periacetabular bone metastases commonly cause severe pain and adversely affect the activities of daily living [8]. In such instances, non-surgical treatments such as radiotherapy and bone modifying agent are generally applied as initial bone management. However, Callstrom et al. reported that pain relief was not obtained in 20–30% of cases after radiotherapy [9]. In our institution, 25 Gy of irradiation dose was commonly performed for bone metastases from lung cancer to relieve pain. However, there have been some cases in the literature in which the pain relief was insufficient. If the symptoms do not improve with these treatments, minimally invasive intervention or surgical treatment may be considered. However, the appropriate treatment for painful periacetabular metastasis after non-surgical treatment remains controversial and should be carefully planned based on its location, size, and the oncological condition of each patient [10]. In patients with a predicted long-term prognosis, for example in patients with breast cancer, thyroid cancer or cases with oligo-metastasis, tumor resection surgery using a hip prosthesis with a reinforcement ring, otherwise known as the Harrington procedure, are preferable [11]. Less-invasive treatments including percutaneous cementoplasty, radiofrequency ablation, cryoablation are generally indicated for cases with an estimated poor prognosis [12,13,14,15].

While hip prosthesis surgery can maintain long-term pain relief and preserve lower limb function, it is usually highly invasive with a high complication rate [16]. One systematic review reported that the postoperative mortality rate of periacetabular metastasis was 3.3% and the complication rate was 19.5% [17]. In another series, the mortality rate within 3 months after hip arthroplasty was 7% [18]. The use of such procedures needs comprehensive discussion especially for patients with advanced cancer, poor general condition, and poor prognosis.

The Harrington procedure is a method in which Steinmann pins are inserted retrograde from the acetabulum into the ilium, cemented together with the acetabular component, and followed by a total hip arthroplasty. As this procedure is technically complicated and not suitable for patients with advanced cancer, the modified Harrington method was developed, in which the direction of the pin was changed and/or screw/plate fixation was used and is recently more commonly reported. Although postoperative limb function and pain improvement are considered to be ideal, a wide range of complication rate has been reported at 6–53% based on systematic reviews [19,20,21]. Hence, the indication of this procedure also requires careful consideration [22].

In contrast, percutaneous cementoplasty for periacetabular bone metastasis, classified as a minimally invasive treatment, involves percutaneous injection of low-viscosity bone cement into osteolytic metastases [6, 7]. It has an ability to acquire immediate skeletal strength and provide pain relief. In addition, the skin incision is minimal and infection risk becomes very low. The reported mean Numerical Rating Scale (NRS) is significantly reduced from 6.1 points before surgery to 3.2 points at 1 week and 2.1 points at 1 month after the surgery. Postoperative gait function was improved in 28% of patients and 40% of the patients maintained this function. As for complications, extraosseous cement leakage was seen in 36% and pulmonary cement embolism in 11% [23]. There have been many reports on percutaneous cementoplasty, and most of them have similar results on good pain relief [24,25,26]. However, if a lesion has a high amount of collagen matrix, it is sometimes difficult to inject a sufficient volume of bone cement [27]. The cement volume that can be injected percutaneously is reported to be approximately 30% of the bone defect and the volume does not increase even after radiofrequency ablation [28]. It has been reported that cases with insufficient cement filling tend to experience less pain relief [29]. The durability of percutaneous cementoplasty is still controversial. The duration of pain relief has been reported to range from 7.3 months to more than 1 year [23, 30]. Even if pain relief is acquired initially, insufficient cement filling may be a cause of its low durability. Therefore, in cases with bone lesions occupied with solid and collagenous tumor matrix, percutaneous cementoplasty might not be suitable and open curettage and cementation surgery would be necessary. However, metastases from thyroid, kidney, or hematopoietic carcinoma tend to be less collagenous and more hemorrhagic, and consequently percutaneous cement injection may be preferable. [31].

While simple open curettage and cementation surgery has been a popular procedure for skeletal lesions and has also been demonstrated in many metastatic cases, this procedure for periacetabular lesions is still not routinely performed [32,33,34]. An article has reported 35 periacetabular metastases and 26 curettage and cementation surgeries that were performed with an average operating time of 168 min and a total of 3,150 ml of estimated blood loss [35]. This highlights the difficulty of the curettage surgery for this lesion.

There is a report of a cementoplasty and screw fixation performed through a 3-cm skin incision, and it was termed “minimally invasive” [36]. As our two cases required a 4-cm skin incision and 1 cm of cortical fenestration, we have termed it “mini-open” to indicate that it is more than minimally invasive but significantly less than the standard open method.

MO-PAC uses an anterior approach, through a small skin incision and a split of the iliopsoas muscle to directly reach the anterior site of the periacetabulum over a short distance. A small cortical fenestration of about 1 cm is then made at the anterior inferior iliac spine, allowing the tumor to be scraped off and sufficient cement to be injected. The major advantage of the anterior approach is that it provides easier access to the lesion compared to the lateral or posterior approaches. Disadvantage is that if the lesion is small and located at the posterior column, the anterior approach may compromise the mechanical strength of the periacetabulum. A lateral approach has a disadvantage of requiring a larger skin incision due to separation of the thick gluteal muscle and a greater distance to the periacetabulum. MO-PAC can be carried out with only an image intensifier guide and does not require intraoperative computed tomography (O-arm). Therefore, it can be performed in most standard medical facilities.

MO-PAC is a surgery that may be classified between percutaneous cementoplasty and simple open curettage and cementation surgery. Although there may be many cases that have undergone similar procedures in other medical facilities, to our knowledge, there have been no reports that clearly describe this procedure in detail. The significance of the naming of MO-PAC is to remind surgeons that there is another option of cementation surgery for the periacetabular metastasis. Cases of Harrington class I will be good candidates. Our two cases underwent this procedure without preoperative embolization, because hemorrhage during curettage was estimated to be minimal due to the solid nature of lung cancer tissue and preoperative irradiation fibrosis. However, in bone metastasis with abundant blood flow, such as renal cell carcinoma, hepatocellular carcinoma, and thyroid cancer, it is necessary to evaluate the blood flow of the periacetabular bone metastasis by contrast-enhanced CT before surgery, and preoperative embolization is indicated if necessary. We have been indicating the modified Harrington procedure for patients who are highly active, in good general condition, and with estimated long-term survival. MO-PAC is preferred for patients with short-term prognosis and/or poor general condition. However, as we only have the experience of two cases, the long-term durability of this procedure cannot be generalized. But still equal or longer durability is expected as that of percutaneous cementoplasty.

In conclusion, MO-PAC is within a spectrum of curettage and cementation surgery, and sub-classified as a type of minimally invasive surgery. We currently recommend this procedure for patients with painful periacetabular metastasis after radiotherapy, Harrington class I, low activity, and an estimated short prognosis.