Abstract
Introduction
The published results of the use of a dual mobility cup to prevent instability in primary and revision total hip arthroplasty (THA) have established its efficacy. However, the monoblock, porous cobalt chromium cup design makes secure fixation difficult to achieve, limiting its use in patients with significant acetabular deformity or bone loss. Recently, a modular version of the dual mobility cup was introduced, consisting of a conventional porous shell with holes to allow augmented screw fixation, a highly polished modular metal liner, and a standard bipolar femoral head. The purpose of this report is to present its various indications, the surgical technique, and report our initial results.
Methods
With IRB approval and FDA clearance, we implanted the modular dual mobility (MDM) cup in 15 patients undergoing primary and 5 patients undergoing revision THA deemed high risk for instability. Indications included septic and aseptic revision surgery, developmental hip dysplasia, avascular necrosis, recurrent dislocations, hemiarthroplasty conversion to THA, periprosthetic fracture, abductor insufficiency requiring augmented repair, and hypermobility from auto-immune inflammatory disease.
Surgical Technique
The acetabulum is prepared in the standard fashion for implantation of a press-fit component. After implantation and possible screw augmentation, osteophytes are removed. A modular metal liner is manually inserted into the shell by lining up tines and then impacted into place. Concentric positioning must be confirmed. After standard femoral stem preparation, a dual-mobility head with multiple neck length options is easily assembled and placed on the trunion. The hip is then located and assessed for limb length, stability, and offset.
Results
In the 15 primary THAs, successful implantation of the MDM construct was accomplished without issues related to the aforementioned technique. Adjunct screw fixation was utilized in 8 patients based on initial rim fit and bone quality. In all cases, the hip had to be manually dislocated because of increased stability. There were no peri-operative complications related to the MDM.
In the 5 revision cases, insertion was possible in 4 of 5 patients. In 2 cases, the MDM liner was used in previously implanted, well-fixed and positioned metal acetabular shells compatible with the MDM insert. In 2 cases, the original metal cup was replaced with a shell compatible with the MDM insert. In the remaining patient, a failed hemi-resurfacing, the use of the MDM was abandoned because of impingement and excessive lengthening causing the inner trial head to disassociate from outer trial head.
Discussion
The MDM cup offers a number of important features not available on the original dual mobility designs. These include the use of: 1) a conventional shell, inserted with familiar instrumentation; 2) a shell that can be used with either a highly cross-linked polyethylene liner or the modular polished metal liner; 3) conventional cancellous screws that makes possible augmented fixation in cases of significant bone loss or acetabular deformity. These features make possible the use of the dual mobility concept without the need to add to a hospital's cup inventory. The initial results in a variety of primary and revision conditions have been encouraging.
