FUNCTION OF A TOTAL HIP PROSTHESIS
CONTENTS
piston-like movements in the total hip
range of movements in the total hip / impingement
leg length & muscle tension & offset
THIS CHAPTER IS UNDER REWORKING- 13/03/2007 - be patient
STABILITY OF THE TOTAL HIP
In the healthy hip joint the femoral head is continually in close and stabile contact with the socket during all movements.
The stability of the healthy hip joint is provided by by numerous supporting structures around the hip joint, including a thick joint capsule, a system of joint ligaments built in the joint capsule, and a ligament inside the hip joint itself. These joint structures create a passive resistant force on the hip joint that keeps the femoral head in close contact with the hip joint socket during all movements.
Moreover, the 19 muscles surrounding the hip joint provide further dynamic stability to the hip joint. Every surgeon who tried to extract the femoral head from the hip joint in a patient with a broken femoral neck (collum femoris fracture) knows how difficult task it is.
On the other hand, during total hip replacement a portion of these supporting structures (muscles, ligaments, capsule) is cut ( divided) for easier access to the hip joint. Even if the surgeon tries to restore muscle and soft tissue balance by suturing together the cut ligaments, muscles, and joint capsule after the total hip replacement, there is usually some imbalance of soft tissues left.
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Picture: Stability of the healthy hip joint and stability of the total hip joint.
Click on the icon for a full size picture.
Upper picture: In the healthy hip joint ligaments, joint capsule and muscles around the joint (and one ligament inside the joint) provide continuous close contact between the femoral head and joint socket. The femoral head is large.
Lower picture: During the total hip joint operations a portion of the muscles, joint ligaments and joint capsule were severed to gain access to the hip joint. Even if these structures were sutured back after insertion of the artificial hip, the force which keeps the ball component in close contact with the cup component has been impaired. Note also that the size of the ball component is smaller than the size of the removed head.
"Pistoning" (=piston-like) movements of the ball component
X-ray studies of patients with total hip joints demonstrated that the ball component separates from the center of the cup component during gait.
When the operated on leg swings (the hip is not loaded) the femoral ball moves to the upper outer side of the cup component.
When the patient’s leg comes in contact with the floor (the leg takes the body's weight) the ball returns to the close contact with the whole cup.
Thus, the ball moves from the center of the cup to the outside of the cup and then backs like a piston. The "pistoning" movements are small, between 2 to 5 millimeters. Studies showed that these "pistoning" movements occur in all conventional total hips where the metallic ball articulates with polyethylene cup (Dennis 2001) and in total hips with ceramic bearing surfaces. The "pistoning" movements were not observed in metal on metal total hips (Komistek 2002).
Picture: Pistoning movements of the ball component
Click on the icon for a full size picture
Left side: During stance phase of the gait, when the leg is hitting the ground and the total hip is fully loaded, the ball component is in close contact with the whole cup component.
Right side: during swing phase of the gait, when the leg is swinging in the air, the total hip is not loaded with the body weight. The ball component moves out of the polyethylene cup component some millimeters. The ball strikes a small area on the periphery of the cup and the wear in this area increases.
It is important to realize that these piston-like movements are very small, only about some millimeters, although the patients feels / hears them very distinctly.
In laboratory these movements and squeaks are observed mainly in ceramic total hips. Ceramic surfacece are hard, smooth and repulsing water. Thus, they separate easily; consequently, the pistoning movements and sound are not observed on metal on metal joint surfaces that are wetted easily.
Simulation of this "pistoning" motion of the ball in the cup in laboratory produced loud squeaking noises. (Stewart 2003)
What is the practical importance of this small pistoning movement?
First, it explains the clicking noises that many patients are feeling.
Clicking or squeaking in the total hip joint is a common complaint early after total hip replacement. If it is not painful, the surgeons dismiss these symptoms and tell the patient that the clicking or squeaking is caused "by a tendon or by a scar tissue, moving over the new total hip".
Actually, these noises may be rather caused by the pistoning movements of the ball component.
Are these noises and pistoning movement a bad sign? Nothing point to this. Moreover, when soft tissues mature and muscle force returns, these noises and movements cease in most patients.
As yet, this is so recent discovery that one may only guess on its importance.
Edge load and stripe wear
Another peculiar movement between the cup and ball component occurs when patients are climbing steps (stairs) or rising from chairs. Statistics demonstrated that total hip patients are performing on average 45 stair climbs and 70 chair rises every day.
Patients climbing stairs, or rising from chairs must stretch forcefully the thigh that has been bent up at least to 90 degrees (depending on the height of the chair or step) to rise. When the thigh is bent the ball component is in contact with the back edge (rim) of the cup. When the patient then stretches the thigh, the ball is first forcefully pushed against the back edge of the cup. (see the next picture "The 90 degrees rule") before it begins to rotate. The stress is at this phase concentrated on a narrow area of the edge of the cup. This is so called "edge loading" when the ball is in contact with the cup in a small area. The high stresses in this stripe-like area cause "stripe wear" on a small edge area of cup and on the corresponding surface of the ball. The stripe wear is easily observed if the surfaces are very hard (ceramic), on surfaces made from softer materials (polyethylene or even metal) the self polishing soon obliterates these stripes. (Walter 2004)
References:
Dennis DA et al. J Biomech 2001; 34: 623-29
Lombardi AV et al. J Arthroplasty 2000; 15: 702- 9
Stewart TD et al. J Arthroplasty 2003; 18: 726 – 34
Walter WL et al.: J Arthroplasty 2004, 19: 402-13
RANGE OF MOVEMENT IN THE TOTAL HIP
impingement
Picture : The 90 degrees rule & impingement
Click on the icon for a full size image
The total hip prosthesis is constructed so that it will allow the patient to flex the thigh from 0 to 90 degrees against the trunk (Upper picture). This range of movement must not cause impingement (encroachment) of the neck against the rim of the cup. The total hip is stable within this range of flexion (bending the thigh forward).
Show Picture: Impingement of the hip joint
Click on the icon for a full size image
Flexing the thigh beyond 90 degrees (lower picture) will bring the neck of the femoral prosthesis against the rim of the cup. The neck of the stem component impinges on the rim of the cup. This contact produces a hinge that lifts the ball out of the cup. Besides that this position forces the ball component out of the joint, it also damages mechanically the rim of the cup. This impingement may be a cause of fracture if the cup is made from ceramic. (See Ceramic total hips)
In this position the ball of an ordinary total hip prosthesis begins to leave the cup and is prone to dislocate. There is no strong joint capsule to keep the ball in place. This is why the patients are urged not to bend the thigh beyond 90 degrees against the trunk.
Theoretically, large diameter ball allows more flexion than the small diameter ball. See the picture
Picture: Large ball is more stable than a small ball
From the picture it is also apparent that the smaller the diameter of the ball component the less stable is the total hip joint. The impingement of the neck against the rim of the cup appears already at about 70 degrees of thigh flexion in the small ball total hip, whereas the large ball total hip allows flexion to about 110 degrees.
Manufacturers are now offering total hip prostheses with large diameters (> 30 mm) of ball components. These prostheses allow flexion above 90 degrees without impingement of the neck against the cup's rim. The advantage of these prostheses thus would be greater stability of the total hip joint. As yet, this combination is possible with metal-on-metal total hips (See the chapter Dislocation of the total hip)
In theory, however, the larger the diameter of the ball component, the greater the wear, especially if the bearing couple is polyethylene cup against metallic ball component. This is so because the wear depends on the distance travelled between bearing surfaces.
On the other hand, some scientists maintain that the wearing mechanism (lubrication) is more favourable with large ball-cup combination. Thus, (theoreticall) the wear should not be increased in total hips with large balls.
You may choose which argument you like.
3 MUSCLE TENSION & LEG LENGTH & offset
The tension of the muscles around the total hip prosthesis is the only active force that keeps the components of the artificial hip joint together. The length of the prosthetic neck regulates the tension in the muscles around the total hip joint. The longer the neck section, the greater the tension in the muscles and the greater the force that keeps the ball in the cup. The surgeons always strive to get good tension and balance in the muscles around the new total hip.
On the other hand, the longer the neck section of the shaft, the longer the leg will be. The operated leg may be too long if the surgeon was forced to use an ordinary shaft component with a long neck to balance the muscle tension properly. This may happen in patients with previous operation on the tissues in the hip area.
The surgeon may also to use a femoral component prosthesis with a longer neck that is, however, placed "more horizontally". Such femoral components are called components with "large lateral offset". Femoral component with large offset tightens the soft tissues without causing leg length inequality.
Picture: Prosthetic neck's length, offset, muscle tension, and leg length.
Click on the icon for a full size picture.
The slack muscles around a total hip joint ( left side in the picture) may be stretched in two ways.
1 ) The surgeon uses a femoral component with a long neck section. (Upper picture). Femoral component with long neck will stretch the slack tissues, pushing them "downwards". The result is tight muscles around the total hip but also longer leg on the operated on side.
2) The surgeon uses a femoral component with long offset. (Lower picture)
Offset is the distance between the ball and the shaft's longitudinal axis.
A femoral component with a long offset thus has a long neck that is, however, pointing more horizontally. Thus, although the femoral neck is longer it does not change the length of the leg. The femoral component with large offset stretches the slack muscles "sideways".
Other (theoretical) advantages of a femoral stem with a large offset are:
greater range of motion
increase in lever arm and thus increase in muscle power
Disadvantages (possible ) of a femoral stem with a large offset are:
more stresses on the neck component of the prosthetic shaft (not important with modern metal alloys)
more stresses on the skeleton on the inside of the thigh bone and possible increased risk of loosening (McGrory 1995)
In practice, however, the offset cannot be increased to much; thus very large leg length differences cannot be completely equalized by choosing prosthesis components with large offset.
See also the chapter Too long leg
MODULAR SYSTEM
The surgeon thus needs a choice of shaft prostheses with different lengths of the neck section. Many sizes of shaft prostheses, each with different length of the neck, require large inventory. The manufacturers thus developed a modular system for femoral shaft components.
Modular head and shaft of a TH.
Click on the icon for a full size image.
In modular system, the prosthetic shaft ends with a universal conical taper (or a cone). To this cone there is a series of exchangeable ball components, each with a bore hole of different length. The longer the bore hole, the more the ball slides on the taper and the shorter the neck of the prosthesis. At operation, the surgeon chooses the ball with appropriate length of the bore and then inserts it on the cone with a "gentle" tap. The tapered interlock then fixates the ball to the shaft component. The strength of this interlock increases by repeated blows against the ball that are generated during walking.
The tapered interlock system that fixates the ball to the stem cone is also called Morse taper. If carefully assembled during operation, the strength of this interlock is sufficient good and the components will not disassemble later. For more details on Morse taper lock go to the chapter Mechanics of Morse taper.
Also the shaft may be modular, composed from parts of different size and shape
Picture
Modular shaft components: balls and shafts of different sizes
Click on the icon for a full size image.
Note that also the shafts are modular, the surgeon can assemble shafts of different length, offset, etc.
The number of modular components in such a system varies, manufacturers offer "boxes of bricks" containing up to 60 individual components. The surgeon assembles the shaft of right proportion from these components at the operation table.
The cup components of cementless total hips are all modular, consisting of the metallic back up envelope and the polyethylene or ceramic liner (inlay). Again, the fixation of the liner to the metallic envelope (shell) may be by (Morse) taper or by another patented mechanism. See the chapter Cemented and cementless THP.
Advantages of the modular system:
The ball and the shaft may be made from different materials. Shafts from Titanium alloys may be coupled with balls made from Cobalt-Chrome alloys or ceramic.
The surgeon has the choice to use a shaft component which is almost individually adapted to the shape of the patient's thigh. This is advantageous for revision operations, where the ordinary shape of the marrow cavity in the thigh bone is destructed by osteolysis (bone dissolving disease). An ordinary prosthetic shaft would not fit to the large dimensions of the cavity left after failed total hip prosthesis.
Disadvantages:
The taper junction may produce wear particles and corrosion products (soluble salts of the metals) . The ions (dissolved metals) of Cobalt, Chromium, or Titanium enter the circulation and rise the concentration of these metals in the blood.
The ball may lose its coupling with the taper and dislocate.
Stress shielding effect of the shaft component
"
Stress shielding" – you will probably hear this term often from your surgeon and your surgeon will often hear this term from the manufacturer of the total hip device.What is it this stress shielding? Basically, it is a mechanism that protects the skeleton from the natural stresses that the everyday life puts on it. Total hip device exerts such stress shielding effect on the skeleton around it.
We start from the beginning.
The skeleton in our bodies is loaded; the skeleton of the lower limbs sustains the load from the body weight, but not only that. The muscles attached to the long bones exert stresses on the skeleton.
The skeleton is very economical system. Where the weight load on the skeleton is large, the skeleton grows more bone tissue in the loaded area; the net result is a more closely packed and stronger skeleton that has the strength to sustain the increased load.
In areas with diminished load the skeleton retains only so much bone tissue that is necessary to sustain the diminished load. The skeleton in the unloaded areas is weaker.
A simple mechanical rule says that in every composite system composed of two materials where one component is stiffer, the stiffer component will sustain the greater part of the load.
Femoral shaft component placed within a thighbone makes such "two materials composite" where the femoral component sustains the greater part of the load.
The shaft component of a total hip device is much stiffer than the skeleton and will take the greater part of the body weight load. Consequently, the shaft component is "overloaded", whereas the skeleton around the shaft is "unloaded".
Picture: Simple scheme of stress shielding.
Click on the icon for a full size picture
Left picture:
Imagine the stresses from the body weight on the lower limb skeleton as a steady flow of impulses that starts in the lower back. In the normal healthy skeleton, the stresses flow symmetrically from there downwards through both hip joints, thighbones, knee joints, lower leg bones, and feet into the floor.
Right picture:
The situation changes when there is a total hip joint device. The much stiffer shaft component of the total hip takes over the majority of the load stresses.
Now the stresses of the body weight flow through the total joint center and then through the shaft component of the device. The flow of the stresses then enters the thighbone at the tip of the femoral component. The consequences of the changed flow of stresses are two:
First, the upper part of the thighbone is unloaded; it contains less bone tissue and it is weaker, more susceptible to fracture. In the right hand picture the stress shielded area is whiter.
Second, the skeleton around the tip of the femoral component is overloaded; it becomes more thick and stronger. In the right hand picture this area is darker. Unfortunately, the thickening of the skeleton is often painful. The patients with cementless shafts of total hip devices often claim about the pain in the thigh, especially during the first years after the surgery.
The surgeons believe that stress shielding is harmful because the weaker skeleton may fracture. The manufacturers are developing shaft components that have less "shielding effect". Because the stress shielding effect depends on the difference between the stiffness of the shaft component and the stiffness of the thighbone, the manufacturers try to produce shaft components with stiffness values more close to the stiffness of the thighbone.
"Diminished shielding effect" of the femoral shaft component is one of the selling arguments of new models of total hip devices. Can one really produce artificial total hips with stiffness values that are almost identical with the stiffness of the skeleton around the total hip device?
Several studies demonstrated that this is an impossible goal. Changing the geometrical form and the material of the femoral components, the manufacturer may produce femoral component with stiffness that is perhaps only twenty times greater than the stiffness of the thighbone, but the stiffness of the component will never come really close to the stiffness of the normal skeleton.
References:
McGrory et al.: J Bone Joint Surg-Br 1995; 77-B:865-9
Pennock J et al.: Morse type tapers. J Arthroplasty 2002; 17: 773-8
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Revised Sept 2004