When dealing with massive periprosthetic osteolysis the aims of surgery are usually to restore bone stock around the arthroplasty, gain stable implant fixation, restore the joint mechanics and reduce the particle debris load . This was successfully accomplished in the patient's contralateral knee which underwent a full revision knee arthroplasty with stemmed implants and extensive bone-grafting in 2000, following severe polyethylene delamination, osteolysis and component loosening. However, treatment decisions are also very much dependent on non-joint patient factors, such as the patient's clinical status and physiological age, comorbidities, and activity levels [8–11], and our patient was deemed no longer medically fit to undergo any kind of extensive or invasive surgery. He thus underwent minimally invasive cyst debridement and grafting alone.
One can argue that the cyst debridements should have been performed in conjunction with a further total synovectomy and tibial polyethylene spacer exchange, in order to remove any residual particulate material and the wear particle generator . Indeed modular polyethylene exchange is a reasonable option when dealing with polyethylene wear and osteolysis in patients with well-fixed knee components [12, 13], without evidence of accelerated wear or severe delamination , and has similar short-term results and failure rates when compared with patients having full revisions of all the knee components . However, our patient had an arthroscopic synovectomy almost 4 years earlier during which no visible polyethylene delamination or macroscopic wear was seen, and successive knee radiographs had not shown progressive joint space narrowing. Additionally, the patient's presenting symptoms were not consistent with active synovitis, and the authors were keen to avoid the more extensive surgery required for a bearing exchange that may not have been necessary.
Traditionally, bony defects in revision knee arthroplasty have been treated with autologous bone, allograft, polymethylmethacrylate (PMMA) bone cement and implant augments [9–11, 14], however not all these options were open to our patient. Given the minimally invasive nature of the intended surgery, we considered using PMMA as a void filler for our patient's osteolytic cysts. Cementation would have had the advantage of providing immediate stability [15, 16], though would not have functioned as a biological scaffold, and might have caused thermal necrosis of the surrounding bone possibly leading to further osteolysis [17, 18]. Autologous bone grafting was also considered because of its biological advantages, as well as its proven record in dealing with small tibial bony defects , and with impaction grafting in knee and hip revision arthroplasty .
However we decided instead to use Hydroset™, an injectable osteoconductive calcium phosphate bone cement, for its versatility and simplicity of use , and to avoid any additional donor-site morbidity. Hydroset™ is composed of a mixture of tetracalcium phosphate, dicalcium phosphate dihydrate and trisodium citrate, which crystallizes to form hydroxyapatite without an exothermic reaction, reaching 75% compressive strength by 4 hours and full strength at 24 hours . It physically interdigitates with the adjacent bone , and though not designed to provide any structural support, forms a structure that is more mechanically stable than either cancellous bone graft or bone substitute blocks or pellets [22–24]. It has improved manual handling and mechanical properties when compared with other calcium phosphate and calcium sulphate cements [21, 25, 26] and creates a scaffold for osteogenesis which is gradually replaced by creeping substitution .
We were unable to find any data in the literature pertaining to the use of bone cements in the context of osteolytic cysts in knee arthroplasty, however these analogues have been successfully used in the treatment of benign bone tumors, and for hardware augmentation in fracture surgery, and have proven biocompatibility, bioactivity and osteoconductivity [26–30] One study of 13 patients with large expansive (mean volume of 38.5 mls) osteolytic benign bone tumors, (similar in size to our patient's cysts) used composite bioceramic osteoconductive grafts, combining porous hydroxyapatite with calcium sulphate, following curettage and phenolisation. 11 of the13 lesions displayed clinical and radiological consolidation at a mean of 4.6 months, and patients had good return to normal function . Another study of 23 patients with bone cysts or benign bone tumors (measuring a mean of 23 mls) treated with calcium-sulphate pellets with or without added demineralised bone matrix, found complete bone regeneration by 6 months in both treatment groups . A multicenter trial of 46 patients with benign bone tumors (ranging in size from 0.15 to 112 mls), found 96% bony ingrowth and 100% graft resorption at 12 months when using the same calcium-sulphate pellets, with no difference between those patients treated with calcium-sulphate alone or in combination osteoinductive agents . Additionally, a recent meta-analysis of 14 randomized controlled trials using calcium phosphate bone cement for augmentation of metaphyseal fracture fixation (in the tibial plateau, femoral neck and calcaneum), found that patients had a lower prevalence of pain at the fracture site and a decrease in the loss of fracture reduction, compared with those patients treated with autogenous bone graft .