The use of minimally invasive approaches in total hip arthroplasty has been a topic of substantial controversy in the last decade. Proponents have typically pointed to more rapid recovery, reduced pain, and avoidance of excessive tissue trauma, while detractors have noted the increased technical difficulty of performing already complex procedures with reduced visibility and have noted higher rates of complications such as iatrogenic neurovascular injury, fracture, and component malposition. Many of the studies in the existing literature have been subjective, retrospective, and nonrandomized. Thus, there is a need for studies with objective measurement of clinically relevant markers. Although true blinded randomization is impractical, investigators should strive for a study design that is prospective and utilizes comparable patient groups.
In their study, Bergin et al. sequentially assessed twenty-nine patients treated with a direct anterior approach by a single surgeon and compared them with twenty-eight patients treated with a standard posterior approach by two different surgeons. Levels of inflammatory markers and creatine kinase (CK), a marker of muscle injury, were assessed preoperatively, immediately postoperatively, and on postoperative days 1 and 2.
The current study is valuable for several reasons. First, Bergin et al. attempted to quantify surgical trauma with a biochemical marker. Subjective measures, such as patient satisfaction, cosmetic appearance, or pain, can be easily biased by patient or investigator expectation. Even putatively objective measures such as gait speed, hip range of motion, or length of hospital stay can be influenced by pre-intervention expectations—put simply, a patient who expects to walk quickly and easily following surgery will typically do better than a patient who expects a slow and painful recovery. Second, the authors compared two clearly different approaches; the majority of studies evaluating objective markers have compared mini-posterior with standard posterior approaches1,2. For example, Suzuki et al.3 assessed inflammatory and muscle enzyme markers in patients who had undergone total hip arthroplasty via a mini-incision or a standard incision, and found less inflammation in the mini-incision group but no difference in creatine phosphokinase levels. However, the deep surgical approach was otherwise identical for their two groups, and it is hard to see how incision length would greatly affect the degree of muscle trauma. In a cadaveric study, Mardones et al.4 compared a posterior approach with a two-incision technique, and visually compared the area of muscle injured. This group found that "damage to the muscle of the gluteus medius and gluteus minimus was substantially greater with the two-incision technique than with the mini-posterior technique. Every two-incision total hip replacement caused measurable damage to the abductors, the external rotators, or both. Every mini-posterior hip replacement caused the external rotators to detach during the exposure and had additional measurable damage to the abductor muscles and tendon." However, it is unclear whether the two-incision approach utilized an inside-out as opposed to an outside-in blind placement of the posterior incision; this could presumably affect the degree of muscle injury.
The study by Bergin et al., however, also illustrates the inherent limitations of applying the gold standard of a randomized controlled trial to surgical procedures. The two groups were similar but not matched; the anterior approach group was 66% female, compared with 50% in the posterior group. In addition, the mean surgical time for the posterior approach was 50% longer (118 versus seventy-eight minutes in the anterior group), which could reflect differential levels of skill between surgeons, the relative ease or difficulty of the surgical approaches, or varying degrees of involvement of resident physicians. In any case, it is clear that these issues represent substantial potential for confounding results. The typical man, even with correction for weight and body-mass index (BMI), has greater muscle mass about the hip than the typical woman, thus predisposing him to a greater rise in CK level. The marked early increase in levels of CK in the posterior approach group may simply reflect the fact that the patients got to the post-anesthesia-care unit later! There is also a lack of clear data in the literature on what happens to CK levels following surgery: there may be some relationship to elevation in the C-reactive protein level, but this is by no means certain. Lastly, CK levels appear to peak at forty-eight hours, as indicated by a literature review, but nothing was measured past forty-eight hours in this current study.
A surgical approach that causes no damage to surrounding muscle is unrealistic. Whether the muscle is stretched, transected, or partially torn, injury will occur. In their study, Bergin et al. provided a starting point for objective evaluation of this muscle injury. Authors of future studies should utilize additional markers, such as lactate dehydrogenase, aldolase, myoglobin, troponin, aspartate aminotransferase, and carbonic anhydrase III (CAIII), as well as markers of oxidative stress, to provide a composite picture of muscle stress5.
In the end, performing minimally, or less invasive, surgery involves thorough evaluation and discussion with the patient about risks and benefits. Both traditional and minimally invasive hip replacement procedures are technically demanding and require considerable experience on the part of the surgeon and operating team. Regardless of what approach is used, the ultimate goal of pain-free function for the patient cannot be compromised for transitory gain. The surgeon must take into consideration the patient's anatomy and be aware of his or her own level of skill.