Becker's ASC Review

48 Executive Briefing Ed Stolarski, MD: Patients expectations following total hip replacement have become increasingly more demanding. Return to a pain free lifestyle with no limitations and equal leg lengths are considered the "norm." Meeting these expectations is dependent on the approach as well as reestablishment of the patients' native biomechanics. Patients' leg length and offset should not be at the mercy of limited stem options including offset. The MasterLoc stem gives me incredible versatility with three offset options within an 11 mm range. Whether it is avoiding increasing native offset of a valgus DDH neck or matching the most extreme varus offset, the MasterLoc stem allows for recreating that patient's native biomechanics. This reduces trochanteric bursitis in the DDH patient and avoids increasing leg lengthening for stability in those patients with significant offset. Another nice option is having the ability to go to the next level of offset in the patient whose operative limb is long or equal pre- op. I can make them equal under flouro and clinically, check stability. If unstable, I can add some offset instead of length. I have also been impressed by the uniform leg length as you increase offset, overall leg length does not change. It's a pure lateral move. Lastly, I have used many flat, wedged taper stems and have always been challenged by the broach implant mismatch. The stem frequently sits proud compared to broach. I can honestly say that problem is virtually eliminated with the MasterLoc Hip System. The matched tolerance between the broach and the final implant is incredibly refreshing. Acetabular Cup Positioning Proper cup positioning is important to joint stability, function and reduction of polyethylene wear. Customary practice is to position the acetabular component in what is considered the "safe zone" of 15 ± 10 degrees of anteversion and 40 ± 10 degrees of inclination. 5 Spinal deformities and subsequent spinal fusion along with degenerative disk disease can increase the risk for dislocation due to spinal stiffness and can narrow the safe zone in regards to acetabular component positioning. 6 Recently, this topic has gained more attention in the orthopedic community along with what may be the best means to treat patients with fixed spinopelvic alignment who need a total hip replacement. It has been suggested that surgeons should consider implant design options, such as large diameter or dual mobility bearings, to improve hip stability in patients with fixed spinopelvic alignment as they may be at higher risk of dislocation. 7 Acetabular implant selection must also be considered for young and active patients who have higher activity levels that may demand greater range of motion. Larger diameter femoral heads improve hip joint stability by providing a larger jump distance before dislocation and increased ROM prior to impingement that can lead to lever-out dislocation. 8 Double mobility acetabular components were introduced in France in the 1970s to reduce the risk of THA dislocation in patients undergoing primary THA. Double mobility components increase the head- neck ratio and increase the arc of motion before impingement. 9,10 In recent years, double mobility cups have gained wider acceptance in the United States to address joint instability in primary and revision THA. The addition of highly crosslinked polyethylene to double mobility implants has provided improved polyethylene wear characteristics, making it a preferred option for younger, active patients. 11 Double mobility cups can take the form of monoblock cups or modular cups with hard-bearing inserts. The monoblock version provides a greater ROM and larger jump distance than the modular version. The modular version can allow for screw holes while providing secondary fixation that may be a benefit in lesser bone quality. However, some concerns of higher levels of metal ions have been reported with modular double mobility cups when dissimilar metals are mated to create the hard-bearing cup. 12,13 The Mpact® DM from Medacta is a hemispherical, monoblock double mobility cup. It features a Mectagrip coating that has a high coefficient of friction for initial stability. 14 The Mectagrip coating has open pores up to 350 µm in dimension that offers a favorable environment for bone growth leading to long term fixation. 15, 16, 17, 18, 19 The articulating liner is HIGHCROSS polyethylene that provides a low wear bearing surface. The HIGHCROSS liner can be combined with a BIOLOX delta ceramic femoral head for younger, active patients. "...the MasterLoc stem allows for recreating [the] patient's native biomechanics."

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