The Case for Using Navigation in THA

    Dr. Joseph Moskal reviews his rationale for adopting computer-aided surgery for his total hip arthroplasty procedures.


    Joseph T. Moskal, MD, FACS


    Why should you consider computer-aided surgery (CAS, aka navigated surgery) for your total hip arthroplasties (THA)?

    Let’s start by looking at dislocation. There are two issues with dislocation:

    • How often does it occur (dislocation rate)?
    • How often does it cause revision (revision due to dislocation rate)?

    According to U.S. Medicare data, dislocation rates during the first 6 months after primary THA and revision THA are 3.9% and 14.4%, respectively. [1] Also using Medicare data, Kurtz et al have shown that 7% of primary THAs will be revised within 7.5 years. [2] Registry and Medicare data indicate that the rate of revision due to dislocation can vary from 16% to 34% (Figure 1).

    Figure 1. Percentage of revisions due to dislocation. [3-8]

    Next, let’s consider two innovations in THAs that occurred at about the same time but that influenced the procedure in different ways.

    Larger Femoral Heads

    Larger femoral head sizes were introduced to combat dislocation. With increasing femoral head size, the rate of dislocation at 6 months decreased from 4.21% to 2.14%. [9]

    However, larger femoral head sizes may lead to stretching of the soft tissues, which can cause pain and leg length discrepancy. Why worry about leg length discrepancy? Patient dissatisfaction with changes in leg length is the most common reason for litigation against orthopaedic surgeons. [10]

    The following figures illustrate cases of changed leg length following THA. The first three figures (Figures 2-4) show conventional THA, and the fourth figure (Figure 5) shows an MIS THA without CAS.

    Figure 2. This 74-year-old woman worked for 54 years, right up to the day before surgery. Preoperative x-rays for primary right THA. The patient’s main complaint was pain. There were no predisposing conditions for poor outcomes.

    Figure 3. Same patient as in Figure 2, two weeks post-THA. The patient presented non-ambulating (wheelchair), components were malrotated and malpositioned on x-ray examination. The patient also had 3-cm leg length discrepancy.

    Figure 4. This 60-year-old woman takes care of her 88-year-old mother. She has been a high school teacher for 38 years. Preoperative x-rays for primary right THA.

    Figure 5. Same patient as Figure 4, THA was revised 2 weeks postoperatively with a contrained liner because the patient dislocated every time she got out of bed. She was ambulating a short distance with a walker and had a 2-cm leg length discrepancy.

    Alternative Bearings

    Alternative bearings – meaning not metal on conventional polyethylene – such as cross-linked polyethylene, ceramics, and metal-on-metal, were introduced to combat wear debris.

    Alternative bearings are position-sensitive and less tolerant of variation in component position, especially excessive lateral opening and anteversion of the acetabular cup and liner. This is problematic because acetabular cups implanted using mechanical instruments are not always in the correct position. Published rates of malpositioning range from 62% to 78%. [11,12]

    Most complications and causes for revision in THA are now due to human error. Since the Industrial Revolution, machines (and, more recently, computers) have been used to improve precision and accuracy, thus mitigating human errors.

    I argue that in THA, computers can:

    • Help decrease complications by reducing dislocation and leg length discrepancy
    • Improve THA function by restoring hip biomechanics with the correct center of rotation and offset for each individual patient
    • Improve patient satisfaction by meeting their expectations more consistently

    Perception and Science

    As orthopaedic surgeons, our perception is that all – or nearly all – of our THA patients are doing great. However, science, as evidence-based medicine, demonstrates that all of our THA patients are not doing great.

    Why is component placement so important? Look at the example in Figures 6 and 7.

    Figure 6. This 64-year-old female who had a right THA in September 2008 and a left THA in November 2009. This x-ray is 3 weeks post-THA. Note: both surgeries on one patient, one surgeon, and one hospital.

    Figure 7. Same patient as Figure 6, after revision. Placement is important, left THA required extensive acetabular reconstruction.

    Acetabular Cup Placement without CAS

    Bosker et al demonstrated that freehand placement of acetabular components (n=200) is not reliable. In their study, nearly 80% of acetabular components were not placed within 5° of the goal (Figure 8). [13]

    Figure 8. Percentage of acetabular components within the “safe zone”. [13]

    Callanan et al used a hospital joint registry (n=1,823) to show that malpositioned cups were a rather common occurrence when mechanical guides were used (Figure 9). [11]

    Figure 9. Percentage of acetabular components within acceptable range. [11]

    Acetabular Cup Placement with CAS

    Beckman et al determined that navigation reduces variability in cup positioning and the risk of placing components beyond the “safe zone.” [8] Although they found that the mean cup inclination and anteversion were not significantly different in CAS versus non-CAS THAs, the outliers (components outside the “safe zone”) were minimized. [14]

    The authors’ evidence-based analysis of navigated versus non-navigated THAs found tighter control of the anteversion angle and the abduction angle and fewer dislocations with navigation. [15].

    In agreement with Beckmen et al, there were no statistical differences in the mean abduction or anteversion angles, yet the outliers were reduced with navigation (Figure 10). [9] The difference in dislocation rates was statistically significant (p=0.0317), with navigation resulting in fewer dislocations (Figure 11). [15]

    Figure 10. Difference in upper and lower limits of 95% confidence intervals. [15]

    Figure 11. Dislocation rate (percentage) in navigated THA versus non-navigated THA (evidence-based analysis). [15]

    Why Advocate Navigation for THA

    The author prefers navigation for THA because it has improved his accuracy and precision. It also assists with intraoperative decision making by providing basic biomechanical information during the procedure, including:

    • Cup anteversion angle
    • Cup abduction angle
    • Change in Hip Center of Rotation
    • Change in Combined Offset
    • Change in Combined Leg Length
    • Range of motion to impingement, instability, and dislocation

    Navigation decreases the offset, the neck length, and the leg length discrepancy in about 25% of THAs (Figure 12). In addition, navigation accurately and precisely couples the preoperative goal with the intraoperative execution.

    Figure 12. Data from S-ROM Offset implants, comparing CAS and non-CAS (data from JTM’s practice).

    What else can navigation do?

    • Helps with accurate femoral component position
    • Helps answer the questions “What should the specific target be for each individual patient? How can hip biomechanics, range of motion, and other outcomes be improved?”
    • Helps the surgeon consider the influences of pelvic position (pelvic tilt) and hip rotation
    • Helps with understanding the difference in pelvic positioning from a variety of activities (lying on the surgical table, walking, standing, sitting, stair climbing)
    • Helps with understanding of the change from preoperative pelvic tilt to postoperative pelvic tilt

    Patient-Specific Needs

    Each patient has specific needs in terms of implant component position. What are the hurdles the surgeon faces?

    • Quantification of the target. What is the “patient-specific target”? What should the specific target be for each individual patient to improve stability, hip biomechanics, and range of motion?
    • Defining “pelvic tilt.” There is no universal understanding of the influence of pelvic tilt on component alignment intraoperatively or postoperatively (Figure 13).

    Figure 13. Demonstrating the variation in pelvic tilt. Posterior pelvic tilt = iliac spines lower than the pubic tubercle. Anterior pelvic tilt = iliac spines higher than the pubic tubercle.

    Quantification of Patient-Specific Target

    What is a “patient-specific target” for acetabular component placement? It is simply paying attention to what the patient’s native anatomy tells you about alignment.

    For instance, the natural alignment differs depending on activities during the years of skeletal development. A woman in a culture that includes a lot of sitting cross-legged will have one sort of hip anatomy, whereas a man in a culture that includes virtually no sitting cross-legged but a lot of standing will have a different sort of hip anatomy. Thus their “patient-specific target” will differ.

    CAS helps the surgeon determine what is going on with the patient’s native anatomy, which helps with implantation suited for the patient.

    Pelvic Tilt

    Zhu et al have quantified the influence of pelvic tilt on anteversion: “As tilt increases, the discrepancy between the anterior pelvic plane and the coronal plane widens, each degree of pelvic tilt (anterior or posterior) changes acetabular anteversion by 0.8 degrees.” [16].

    They also note the anterior or posterior pelvic tilt has the greatest potential to become clinically relevant for affecting cup position when it exceeds 10°.

    Surgeons are taught to align the acetabular component with landmarks, such as the plane of the operating table and the presumed position of the pelvis. [11]. Navigation systems that rely on the pelvic anterior plane convert cup alignment values to familiar target values with good accuracy and reproducibility. [17]

    Thus, CAS helps the surgeon account for pelvic tilt intraoperatively.


    We can all do a better job with our THA patients, and we all have room for improvement. Cup position and placement must account for patient-specific needs. Femoral position must take leg-length discrepancy and offset into consideration. CAS helps with both concerns.

    CAS has been proven to be accurate, precise, and safe. It provides more information in “real-time” – during surgery and during component placement – that the surgeon can act on before leaving the operating room.

    Thus, CAS allows surgeons to change “the art of feel” to “the science of numbers.” CAS is not about “visualization”; it is about “localization.” CAS gives the surgeon three-dimensional orientation:

    • When he or she cannot see anatomic structures well enough – for example, difficult surgical exposures, such as with obese patients or with MIS
    • When the orientation of anatomic structures changes during surgery – for example, when a patient’s position changes on the operating table

    CAS and “the science of numbers” will narrow the gap between orthopaedic specialist and orthopaedic generalist.

    Going forward, CAS for THA needs to:

    • Be easier to use
    • Be cheaper (ie, more cost effective)
    • Eliminate arrays and pins
    • Allow better visualization (line-of-sight issues)
    • Include dynamic (functional) patient-specific data
    • Continue to be safe


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