1. BIOMECHANICS

The normal hip joint is best understood as a Lever-Arm (see picture).  The hip center is the fulcrum that sits between the two opposing forces: 

Lever arm #1 is the distance between the hip center and the center of body weight.

Lever arm #2 is the abductor mechanism, which is distance between the greater trochanter (aka the insertion point of our abductors) and the hip center.

These two lever arms oppose each other.  In a normal hip, the body weight lever arm is 2.5 times longer than the abductor, which means that the abductor force must be 2.5 times stronger than body weight.  The greater the body weight, the more force across the hip joint by multiple factors.  This is one theoretic correlation between obesity and hip arthritis.

Joint reactive forces depend on the activity.  Force of walking is 2.5x body weight, force of jogging is 6x body weight, force of running is 8x body weight.

 

 

clinical: In progressive hip degeneration, the head begins to collapse, the neck shortens, and the abductor mechanism loses tension causing weakness and pain due to increased joint reactive forces.  In designing the first modern hip reconstruction, Dr. Charnley wanted to change the forces across the hip center by deepening the acetabulum (shortens Lever arm #1) and performing a troch-slide osteotomy (lengthen Lever arm #2).  But there is no free lunch in medicine and deepening the acetabulum increases bone loss, osteotomizing the greater troch slows rehabilitation and requires greater soft tissue dissection.  Current techniques for hip reconstruction focus on restoring the anatomical geometry of the hip—placing the acetabular component in a low and medial position to restore the center of rotation and selecting a femoral stem that will restore offset (maximizing tension of the abductors) and leg length.

2. HIP ANATOMY

Average adult femoral neck shaft angle is 135, but this is only an average. The neck angle can be measured by placing one line down the center of the femoral shaft and a second line down the center of the femoral neck.   

A high neck angle is a Valgus Hip,  a low neck angle is a Varus hip.  The amount of varus or valgus can also be measured by placing a point at the center of the femoral head (representing the hip center) and extending a line out to the tip of the greater trochanter (see picture).  In a "normal hip" the center of the femoral head should be at the same level as the tip of the greater trochanter, while a Varus Hip the center of the femoral head lies below the tip of the greater troch, and a Valgus Hip the center of the femoral head lies above the tip of the greater troch. 

Importantly the neck angle affects forces across the hip because it changes the tension of the abductor mechanism.  A valgus hip results in improved mechanical advantage to the abductor while a varus hip generates greater forces across the hip center.  

Most femoral components of a total hip replacement rely on bony ingrowth for long term fixation.  Bony ingrowth occurs during the first 6 weeks after a THA, and similar to the fracture healing, it requires micromotion < 50 nm for ingrowth to occur, otherwise, fibrous ingrowth occurs and there is increased risk for persistent motion, early loosening and symptoms of thigh pain.  Thus, for proper ingrowth to occur, the press-fit must be obtained at the time of surgery - the implant must fill the proximal canal and place enough hoop stress on the rim of cortical bone to prevent micromotion.  Both axial (the stem doesnt sink into the femoral canal) and torsional (the stem cannot rotate within the canal) stability are required for initial stem fixation.

The best press-fit occurs when the implant design matches the native femoral canal.  The challenge is the variability of proximal femur anatomy within the population.  The Dorr Classification is the most commonly used classification system for describing the relationship between the proximal femoral anatomy and the emoral diaphysis, otherwise known as the Canal-Flare Index.  This can help with surgical planning to select a stem that will maximize the ability of achieving a press-fit during surgery. In general, as we get older, we lose bone stock, and the metaphyseal region of the proximal femoral canal becomes more and more capacious.  This has implications on the ability to use a non-cemented/press-fit stem. 

More specifically the Dorr Classification describes the relationship of the diameter of the proximal femoral canal relative to the femoral diaphysis.  Its calculated by measuring the ratio between the canal width at lesser trochanter and the canal width 10 cm below the lesser troch.  It actually measures the relative thickness of the cortex by calculating the difference between the full diameter of the bone compared to the diameter of the canal alone (see diagram).

TYPE A femurs have a thick cortex and narrow canal (referred to as a “champagne flute”) and commonly found in men and younger patients.  It permits good cementless fixation with low risk of fracture, however, selecting the proper implant can be challenging given the metaphyseal-diaphyseal mismatch.  A single wedge stem design is often used.  

TYPE B femurs demonstrate some medial and posterior cortex bone loss, and therefore a wider canal, and are an intermediary between Type A and C.  Most femurs are a Type B.

TYPE C femurs demonstrate significant cortical bone loss particularly in the medial and posterior cortex with a capacious canal (referred to as a “stovepipe”), and is common in postmenopausal women and should warn surgeons of concern for cementless fixation particularly difficulty in controlling rotational motion, and increased risk for intraoperative fracture.  

3. CUP PLACEMENT

Placement of the acetabular component depends on the appropriate version, inclination, depth, and height.

The goal of cup placement is to match artificial implant (THA) range of motion with the native hip range of motion.  When these two overlap, there is no impingement, the cup is in the "safe zone".  Impingement occurs when our anatomy wants to move the hip implant further than what is permitted by the orientation of the cup (and stem).  If impingement occurs, its like a car hitting a brick wall: motion comes to a halt, and the hip can either dislocate, or wear out at an accelerated rate.  

We will review the primary considerations for cup placement.

1. Version

Version is the orientation of the implants in the sagittal plane. Neutral version means the cup is facing completely lateral, ie completely in line in the sagittal plane.  Anteversion means the cup is facing slightly anteriorly, this allows for increased hip AD-duction.  In contrast, Retroversion means the cup is opening up slightly posteriorly.  This allows for increased hip AB-duction, but less adduction, and if the hip moves into too much adduction it will impinge on the rim of the cup. 

The target Acetabular Cup version is slight Anteversion.  There is a “safe zone” between 5° and 25° (ie 15° +/- 10°) to match native hip motion and avoid impingement [2, 3].   It can be assessed postoperatively by cross leg lateral XR (a frog-leg lateral gives lateral view of femur, but an AP of the acetabulum).    

There is also Femoral Stem version.  The stem should also be angled slightly forward (anterior), about 15°.  The goal of Combined Anteversion is 35° (+/- 10°) [4].  Studies demonstrate that maximal range of motion before impingement occurs when the acetabular components are placed within this “safe-zone” [5, 6]... which corresponds to the the native acetabular positioning within the pelvis. 

True version of pelvis is angle between a) the line from the posterior to anterior wall of acetabulum (essentially the ilioischial line) and b) a line directly from A to P (this is the alpha angle).  A CT scan gives the best picture of version. 

The expectation for hip ROM after THA is 110 ° flexion and 30 ° extension. Increasing the acetabular anteversion increases the flexion (decreases extension). 

2. Inclination

Inclination is the orientation of the cup in the coronal plane. 

The target acetabular cup inclination is a "safe zone" between 30-50° abduction (assessed by AP of hip). 

Excessive inclination (a vertically oriented cup) decreases the surface area of articulation along the superior dome of the cup and therefore increases contact forces causing higher wear rates (known as edge loading).   Insufficient inclination (a horizontal cup) decreases range of motion (risks impingement with hip abduction), but is generally considered less problematic than too much inclination.

3. Depth

Target is to recreate the anatomic hip center for that patient by reaming to bleeding cancellous bone (assessed by AP of hip: cup should be up to, but not past Kohler’s line, aka ilioischial line).   

Over medializing the cup removes excess bone (always preserve bone, “A.P.B”, is a mantra for TJA surgeons).  However, failure to sufficiently medialize the cup provides less surface area for bony ingrowth, and the cup does not reach the bleeding cancellous bone that promotes ingrowth.  Lastly, recall the force diagram showing the two opposing lever arms converging at the hip center…a lateralized hip center shortens the “abductor lever arm” and thus requires the abductors to work harder to counteract the body weight force acting on the hip.   

4. Height.  

Inferior border of cup should be at the “tear drop” , assessed by AP of hip.  

Clinical Correlate: A high hip center (which is sometimes done intentionally in cases of DDH when there is poor native bone stock around the native acetabulum) affects the abductor muscle tension and increases the joint reactive forces.  

5. Pelvic Tilt

Acetabular orientation is not fixed.  There is a dynamic relationship between the pelvis and the lumbosacral spine, which moves in a predictable way, from standing to sitting, to maintain sagittal balance.

The basic concept is that pelvic tilt and sacral slope are dynamic pelvic parameters (change with body position), but their sum represents the Pelvic Incidence, which is static (always the same number regardless of position).

The Pelvic Incidence is a fixed number in adults, it represents overall body balance.  Yet as we move into different positions (sitting, standing, lying down), the SS and PT change in order to maintain PI.  

When standing, the amount of lumbar lordosis increases (SS increases), so the pelvis tilts forward, ie flexes (represented as a decreasing PT).

During sitting, the Pelvis Tilts backwards (ie extends) causing increased PT.  Increasing Pelvic Tilt Increases Acetabular Anteversion (at a rate of 1.0° pelvic tilt to 0.7° of anteversion) [6]. Increasing the anteversion prevents anterior impingement (its important to avoid anterior impingement bc it causes the femoral head to lever out posteriorly).  During sitting the lumbar lordosis decreases (SS decreases). 

pelvic tilt sacral slope and pelvic incidence

The SS and PT change by the same amount in a normal flexible spine-sacrum to maintain the PI (balance).  Sitting after THA risks posterior dislocation because of anterior impingement, therefore, a mobile lumbo-pelvic segment is essential to minimizing THA dislocation.  However, Lumbar Fusion reduces lumbo-pelvic motion and risks THA dislocation.  The more lumbar segments fused, the less lumbo-pelvic flexibility and the higher the dislocation risk: No fusion: 1.5% risk.  1-2 levels: 2.7% risk.  3-7 levels: 4.6% risk [7, 8].

Lets look more at the measurements to describe the orientation of pelvis [9].

Pelvic incidence (PI) is the main axis of sagittal balance of the spine (ranges 48° - 53° ). It is the angle between Line 1 (perpendicular to Sacral Prominotry, aka superior plate of S1) and Line 2 (center of Sacral prominitory to Hip Center)

Sacral slope (SS) is the angle of the sacrum.  Angle between Line 1 (line parallel to sacral prominotry) and Line 2 (a horizontal reference line)

Pelvic Tilt (PT) is the angle of the pelvis.  Angle btwn Line 1 (center of Sacral prominitory to Hip center) and Line 2 (a vertical reference line). As the upper pelvis moves posterior, the pelvic tilt increases, therefore pelvic tilt can be thought of as the degree of pelvic extension. When the upper pelvis tilts forward (anterior flexion), there is less pelvic tilt.  Pelvic tilt does not change after THA. With normal saggital balance (PI), the PT should be < 22°. 

REFERENCES

1.         Lewinnek, G.E., et al., Dislocations after total hip-replacement arthroplasties. J Bone Joint Surg Am, 1978. 60(2): p. 217-20.
2.         Biedermann, R., et al., Reducing the risk of dislocation after total hip arthroplasty: the effect of orientation of the acetabular component. J Bone Joint Surg Br, 2005. 87(6): p. 762-9.
3.         Dorr, L.D., et al., Combined anteversion technique for total hip arthroplasty. Clin Orthop Relat Res, 2009. 467(1): p. 119-27.
4.         Kosashvili, Y., et al., Acetabular alignment and primary arc of motion for minus, skirtless, and skirted 28-, 32-, 36-, and 40-mm femoral heads. J Arthroplasty, 2013. 28(2): p. 279-285 e2.
5.         D'Lima, D.D., et al., The effect of the orientation of the acetabular and femoral components on the range of motion of the hip at different head-neck ratios. J Bone Joint Surg Am, 2000. 82(3): p. 315-21.
6.         Lembeck, B., et al., Pelvic tilt makes acetabular cup navigation inaccurate. Acta Orthop, 2005. 76(4): p. 517-23.
7.         Perfetti, D.C., et al., Prosthetic Dislocation and Revision After Primary Total Hip Arthroplasty in Lumbar Fusion Patients: A Propensity Score Matched-Pair Analysis. J Arthroplasty, 2016.
8.         DelSole, E.M., et al., Total Hip Arthroplasty in the Spinal Deformity Population: Does Degree of Sagittal Deformity Affect Rates of Safe Zone Placement, Instability, or Revision? J Arthroplasty, 2016.
9.         Phan, D., S.S. Bederman, and R. Schwarzkopf, The influence of sagittal spinal deformity on anteversion of the acetabular component in total hip arthroplasty. Bone Joint J, 2015. 97-B(8): p. 1017-23.


Stem placement

Goals of stem placement are to 1) restore medial offset, 2) restore leg length (read: vertical height), 3) restore femoral anteversion (average 15 degrees).

1. Medial offset should be determined by templating (some patients require standard offset, others require high offset - aka greater medial offset).  The advantage of high offset is that it lateralizes the center of the femoral stem and therefore places greater tension on the abductor mechanism, giving greater stability to the implant.  The disadvantage of high offset is that it can place excessive tension on the abductors and cause trochanteric bursitis.  

2. Vertical height is determined by where the center of the femoral head sits.  Vertical height can be increased if the stem doesn't fully sit properly (referred to as a "potted stem").  Vertical heigh can be increased if the stem is put into valgus (tip of the stem touches the medial cortex of the femur). Vertical height can be increased via +4, +8 etc femoral head (although putting on a +4 femoral head increases the neck length +4, so it affects both the medial offset and the vertical height as a calculation of SOH-CAH-TOA).  

correct stem placement for total hip replacement

3. Femoral Anteversion is about 15 degrees, but the native anatomy of the femur often dictates the version for a press-fit stem (for a cemented stem, or an S-ROM stem, the surgeon has much more control over the femoral version).  Many surgeons use the posterior cortex of the femur, at the femoral neck cut, as their guide for setting version.

measure the femoral anteversion of the stem for total hip replacement

SURGICAL APPROACHES

Direct Anterior Approach

Anterior-Lateral Approach

Posterior Approach