Study Design and Setting
From August 2014 to March 2018, 100 cases of KMLS newly designed cementless stems for the PF, DF, and PT were prospectively registered for use at JMOG-affiliated 14 institutions. Following institutional review board approval, we retrospectively reviewed clinical outcomes of 100 newly designed cementless stems with a minimum follow-up of 2 years in March 2020.
The records of all patients were collected using a questionnaire administered to the members of the JMOG. The collected data included the demographic details, histological diagnosis, surgical stage, adjuvant therapy, size of the prosthetic components, complications, 1-year postoperative MSTS functional score, 2-year ISOLS radiographic evaluation, and oncological outcome at the final follow-up.
The primary endpoint of this study was to identify the prosthesis survival and the implant complication/failure rates. The secondary endpoint was to determine the radiological and functional outcomes.
Demographics, Description of Study Population
A total of 100 cases of KMLS newly designed cementless stems for the PF, DF, and PT fit the inclusion criteria (Table 1). There were 51 males and 49 females. The median age was 49 years. Follow-up was at a minimum of 2 years (mean, 35 months; range, 24–53 months). Four (4%) patients were lost to follow-up. Anatomical locations were as follows: PF (n=49), DF (n=39), and PT (n=12). There were 64 primary tumors (57 bone tumors and 7 soft tissue sarcomas) and 36 metastatic bone tumors. The primary bone tumors included 35 osteosarcomas, eight chondrosarcomas, eight giant cell tumors of bone, five undifferentiated pleomorphic sarcomas, and one leiomyosarcoma. The primary soft tissue sarcomas included two undifferentiated pleomorphic sarcomas, two leiomyosarcoma, two synovial sarcomas, and one liposarcoma. Chemotherapy was administered in 57 patients and irradiation in three patients. Extra-articular resections were performed in seven patients of DF and a patient of PF. For the cementless stems of the PF and DF, straight and curved stems can be used selectively. In the PF, a straight stem was used in eight (16%) of 49 cases. In the DF, a straight stem was used in eight (21%) of 39 cases.
The KMLS system is an original prosthesis of JMOG and is designed especially for patients with an Asian body type (Fig. 1). The metallic parts of the KMLS are made of lightweight and high-strength titanium alloy (Ti-6Al-4V) with good biocompatibility and bio-stability. The base of the stem has a tapered press-fit design, and the interface is processed by porous proofing made of pure titanium to promote bone ingrowth. The cementless stems of the PF and DF have derotational flutes to provide adequate initial rotational stability, and straight and curved stems can be used selectively. This prosthesis has not been approved for that use by the United States Food and Drug Administration.
Variables, Outcome Measures, and Data Sources
We calculated the relative prosthetic-shaft diameter by dividing the diaphysis diameter by prosthetic diameter (diaphysis/stem coefficient) at the midpoint of the prosthetic stem (Fig. 2). It has been reported that the diaphyseal/stem coefficient of cementless stem over 2.5 predicted lower prosthetic survival .
The diaphyseal interface and anchorage were graded by the ISOLS system  at postoperative 2 years; 65 of the 100 were available for ISOLS radiological evaluation (65%).
Functional results were analyzed according to the 30-point functional Musculoskeletal Tumor Society (MSTS) scoring system  at 1 year postoperatively; 72 of the 100 were available for 1-year postoperative MSTS functional score (72%).
Complications or failures after insertion of the KMLS endoprostheses were classified according to Henderson et al.  in five different types: type 1 (mechanical failure due to soft tissue problems, such as debridement, peroneal nerve palsy, dislocation of joint [closed reduction]) and superficial infections), type 2 (aseptic loosening), type 3 (structural failures, such as periprosthetic fractures and hip dislocation requiring surgical treatment), type 4 (non-mechanical failures, such as deep infection), and type 5 (tumor progression).
The statistical associations between the clinicopathological factors and complications were evaluated using the Fisher’s exact test or Chi-square test. The duration of survival was defined as the interval between the date of initial treatment for the primary tumor and the date of death. Patients who died from non-tumor-related causes were uncensored at the time of death in this study. The overall prosthesis survival rate was defined as the time from surgical reconstruction using the KMLS system to revision surgery due to any prosthetic failure including minor parts of the prosthesis, due to local recurrence, polyethylene bushing failure, breakage of the prosthesis, aseptic loosening, or infection. The prosthesis-specific survival rate was defined as the time from surgical reconstruction using the KMLS system to revision surgery due to implant failure excluding infection and tumor recurrence. The limb salvage rate was calculated as the time from surgical reconstruction using the KMLS system to amputation. Survival curves were constructed using the Kaplan–Meier method. The subgroups were compared using the log-rank test. The level of statistical significance was set at p< 0.05 and confidence intervals were reported at 95% (95% CI).
At the final follow-up, 59 patients were alive, and 41 patients had died of disease. The overall survival rates at 2 and 4 years were 88.2% and 79.6%, respectively. Twelve patients developed local recurrence. The limb salvage rate at 2 and 4 years was 98.9% and 96.9%, respectively. Two patients ( osteosarcoma and undifferentiated pleomorphic sarcoma of bone) underwent amputations for local recurrences.