Background 

Minimally invasive total hip arthroplasty (MIS-THA) is often portrayed in the lay community and press as a small skin incision. MIS-THA is actually defined by limited soft tissue and bony dissection. Conventional THA is a successful operation for patients experiencing arthritis of the hip, with a reported complication rate of less than 2% and an implant survivorship of 95% at 10 years.^[1]

The major benefits of MIS-THA are patient perceived incisional cosmesis, psychological patient satisfaction, and possibly a shorter hospital stay.^{[2, 3, 4]}With modern analgesia protocols, MIS-THA patients can often be discharged home within 24-48 hours postoperatively.^{[3, 4, 5]} Decreased blood loss, decreased postoperative pain, and decreased need for postoperative ambulatory walking aids have also been reported with MIS-THA.^[6]

An image depicting minimally invasive total hip arthroplasty can be seen below.

Incision comparison.Preoperative and postoperative gait analysis on 10 patients after mini-posterior THA demonstrated 85% recovery of gait velocity, a 90% recovery of single leg stance time, a 90% recovery of cadence, and a 70% return of stride length as well as less dependence on walking aids.^[7] Moreover, 97% of MIS-THA patients were able to attain standard physical therapy goals (transferring in and out of bed from standing, rising from a chair to standing, moving from standing to sitting, walking 100 feet, and negotiating a full flight of stairs) within one day of surgery.^[5]

However, other studies have found no differences in operative time, pain control, transfusion rates, estimated blood loss, operative time, length of hospital stay, gait (ie, stride length, cadence, or walking speed), or Harris Hip Score.^{[8, 9, 10, 11, 12]}

Indications

Conflicting data and no definitive answer exists regarding whether MIS-THA is an acceptable replacement or even an improvement on conventional THA.

Contraindications

No absolute contraindications exist to MIS-THA; it is defined by limited soft tissue and bony dissection. Patients with a BMI greater than 30 kg/m², severe osseous dysplasia (eg, superior femoral head migration), revision THA, and severe hip contracture or patients who are exceedingly muscular are probably better suited for conventional THA.^{[3, 7, 13]}

Preparation

The preoperative evaluation and preparation for a MIS-THA is exactly the same as for conventional THA, except for specialized surgical instrumentation and implants. Since exposure may be limited, MIS-THA may require modification of conventional instrumentation and possibly additional equipment (eg, fiberoptic light cables, cutaway reamers, Hohmann retractors with light sources, flexible acetabular reamers).^{[14, 15, 16]}

Using particular operating room tables make certain MIS-THA approaches more accessible and may be required to provide requisite traction and permit rotation of the lower extremity. A Judet Orthopaedic Table (Tasserit, Sens, France) or a PROfx Fracture Table (OSI, Union City, CA) can be used with an anterior approach, and the Jupiter Table (Trumpf, Charleston, SC) with the anterolateral approach.^{[15, 17, 18]}

Another unique aspect of MIS-THA is that preoperative templating can guide component and incision size. For example, a 4-inch incision would be appropriate for inserting a size 56 acetabular component while avoiding contact between the prosthesis and skin or subcutaneous fat.^[2] A double-blinded randomized controlled trial identified a doubling of the risks associated with the MIS-THA with a surgeon’s first 60 MIS-THA cases, but this increased risk was not present with a surgeon’s next 60 MIS-THA cases.^[19] This data gives some idea of a learning curve associated with MIS-THA and suggests surgeons not undertake MIS-TKA without appropriate training and mentorship.

Complication Prevention

Complication prevention for a MIS-THA is exactly the same as for conventional THA and should include appropriate preoperative screening and clearance, preoperative templating and planning, and preoperative administration of antibiotics as recommended by the American Academy of Orthopaedic Surgeons.

Approach

Among the first alternatives to the transtrochanteric approach to the hip were the conventional posterior and lateral approaches.^[20] These early alternative approaches sought to limit disturbance to osseous and soft tissue anatomy, thereby avoiding unnecessary complications such as trochanteric nonunion.^{[21, 22]} The conventional posterior and lateral approaches were nevertheless reported to be associated with gluteal nerve injuries.^[23]

Beginning in the 1990s, these conventional surgical approaches were further modified to limit soft tissue dissection and avoid damage to healthy surrounding structures. Minimally invasive total hip arthroplasty (MIS-THA) is actually a broad term encompassing multiple mini-surgical approaches. A single mini-incision approach is usually less than 10 cm in length, while a 2-incision approach usually has 2 incisions, each less than 5 cm in length (see the image below).

Incision comparison.The posterior approach is the workhorse approach for conventional THA.^[20]The mini-posterior approach uses the same plane of dissection as the conventional posterior approach but minimizes the soft tissue dissection.^{[16, 24]} The gluteus maximus split should be kept to a minimum, and release of the gluteus maximus tendon insertion should be avoided. An anterior capsular release is critical. Releasing the anterior capsule facilitates femoral mobilization and easy delivery of the femur into a small wound.^[14] Lastly, a meticulous posterior capsular closure with short rotator reattachment is critical to the mini-posterior and conventional posterior approaches, because it reduces the risk of postoperative dislocation risk from 3% to 0.85%.^[25]

The anterior approach uses the internervous plane between femoral nerve (lateral border of the sartorius and rectus femoris muscles) and superior gluteal nerve (medial border of the tensor fascia latae and gluteus medius muscles). Because no muscle needs to be split or tendon needs to be cut, it represents the optimal mini-incision approach.^{[7, 17]}

For the posterolateral approach, an incision is made over the posterior aspect of the greater trochanter, and the gluteus medius and minimus are elevated off of the capsule.^[26]

For the lateral approach, the skin incision runs from 2 cm proximal to the greater trochanter to 5-8 cm distally along a line parallel to the long axis of the femur. After incising the tensor fasciae latae, the gluteus medius fibers are cut to expose the joint capsule.^[27]

The skin incision for the anterolateral approach goes from the anterior tubercle of the greater trochanter and angles toward the anterior superior iliac spine. Like the anterior approach, it uses the interval between the gluteus medius and tensor fasciae latae and usually only requires elevating the anterior one-third of the gluteus medius.^{[15, 26]} A possible downside to the anterolateral approach is transection of the superior gluteal nerve.^[28]

The 2-incision approach consists of a 5 cm anterior incision directly over the femoral neck used for accessing the acetabulum and inserting the cup and a 3 cm posterior incision in line with the femoral canal used for the femoral preparation. The anterior approach uses the internervous plane between the femoral and superior gluteal nerves, and the posterior approach uses the plane between the abductor tendons anteriorly and the piriformis posteriorly.^[7] Some authors recommend using fluoroscopy to verify the correct location of the skin incisions, while others feel like this extra equipment is not necessary.^{[16, 29]}

Techniques

The femoral neck may need to be osteotomized in situ or piecemeal instead of dislocating the hip and removing the femoral head en bloc.^{[5, 15]}

Complications

Despite the portrayal of minimally invasive totaly hip arthroplasty (MIS-THA) as a small cosmetically pleasing skin incision, plastic surgeons rated 6 of 20 MIS-THA scars as poor and only 1 of 14 conventional THA scars as poor. Thirty of 31 of those same patients also stated that pain relief and implant longevity were more important than scar cosmesis after 2 years of follow-up.^[30]

MIS-THA is associated with a 6% increase in local wound complications.^[9]This may be secondary to increased soft tissue and cutaneous trauma from the retraction or reamers abrading the skin.^[31] However, 2 meta-analyses showed this increase is not statistically significant.^{[11, 12]}

Damage to surrounding structures, especially nerves, is one of the main concerns with MIS-THA. The anterolateral approach places the superior gluteal nerve at significant risk, as 4 of 5 were cut during a cadaveric study.^[28] A meta-analysis demonstrated a 5-fold increase in the risk of LFCN palsy after MIS-THA, as well as a statistically significant increased risk for any nerve palsy.^[12]

Other surgical complications of MIS-THA include proximal femoral fracture (2.8% for 2-incision approach), hematoma (2% from one study), acetabular component malposition (an abduction angle of less than 35° or greater than 50° was seen in 4% and 11%, respectively, using the posterolateral approach), and varus femoral component malposition (14% using the posterior approach).^{[3, 7, 9, 32]} Acetabular cup anteversion was 3 times more likely to be outside the acceptable range with the posterior and posterolateral MIS-THA approaches.^[33]

Since the posterior approach is associated with increased risk of posterior dislocation, it is reasonable to continue to expect this as a potential complication. However, one study of 1,000 hips using the mini-posterior technique reported a posterior dislocation rate of 1.2%, which is less than the value of 5.8% reported using the standard posterior approach.^{[13, 34]} Despite sufficient soft tissue mobilization in properly selected patients, limited surgical visualization may necessitate conversion of the MIS-THA incision into a more conventional THA approach in less than 10% of patients.^[27]

The two-incision approach uses unique dissection planes; as a result, it has a significant learning curve resulting in a higher than expected complication rate.^[35] The rate of complications did not decrease over a surgeon’s first 10 cases, suggesting that a longer learning curve than expected exists.^[36] The 2-incision MIS-THA is supposed to spare cutting muscles and tendons, although cadaveric studies of 10 hips showed that in every case the abductors, external rotators, or both were injured.^[37] Moreover, unlike standard mini-incision approaches to the hip, which use large dissection planes, the 2-incisions have limited extensibility. As a result of the increased level of skill and experience required to perform this approach safely and correctly, many surgeons feel that this approach is best performed by high-volume surgeons who receive specialized training.^[7]

Long-term Monitoring

Patients should be followed at the same time interval as a conventional THA with periodic clinical and radiographic examinations to monitor for aseptic loosening and late infection.

Patient Education

Patient education prior to conventional and MIS-THA with respect to postoperative expectations as well as the administration of anticoagulation are imperative. Counseling with respect to activity level is left to the discretion of the surgeon.