Hip Implants

Dr.S.M.Muhammad Thahir MBBS., DNB(Ortho)., M.N.A.M.S(Ortho)., M.Ch(Ortho).,

INTRODUCTION :• Total hip arthroplasty is an

operative procedure in which the diseased and destroyed hip joint is resected and replaced with a new bearing surface.

• Patients with arthritis can now look to THA with the object of maintaining stability, while relieving pain, increasing mobility and correcting deformity.

• MOST SIGNIFICANT BREAK THROUGH OF THE 20Th CENTURY

• In 1950, JUDET and BROTHERS used acrylic femoral head prosthesis made of methyl methacrylate..

• In 1952 AUSTIN MOORE and FRED THOMPSON independently conceived the idea of fixing endoprosthesis.

• The 1950, WRIST, RING, Mc. KEE-FARRER and others designed the metal on metal total hip arthroplasty but did not prove satisfactory because friction and metal wear

• In 1960, Late Sir John Charnley has done pioneer work in all aspect of THA, including the concept of low frictional torque arthroplasty, surgical alteration of hip biomechanics, lubrication, materials, design and clear air operating room environment.

• Between 1966-1988,Maurice Muller from Switzerland developed a plastic acetabular cup with a 32 mm diameter chromium-cobaltmolybdenum femoral head.

• In 1964,Peter Ring began using metal-to-metal components without cement,

• concept of modular prosthesis developed during 1970

• cementless prostheses came in to picture by mid 1980

ANATOMY OF HIP JOINT

• The hip is one of your body's largest weight-bearing joints.

• Consists of two main parts: • a ball (femoral head) that fits into a rounded socket

(acetabulum) in your pelvis.• Ligaments connect the ball to the socket and provide

stability to the joint• The bone surfaces of your ball and socket have a

smooth durable cover of articular cartilage that cushions the ends of the bones and enables them to move easily.

• Hip joint is unique in having a high degree of both stability as well as mobility

• The stability or strength depends upon : – The depth of acetabulum which is increased by

the acetabulur labrum. – The strength of the ligaments and the surrounding

muscles. – Length and obliquity of the neck of femur which

increases the range of movement

Neck shaft angle or angle of inclination

• It is the angle between the axis of the femoral neck and the long axis of the femoral shaft.

• On average, it is 135 degrees in the adults

Anteversion or angle of femoral torsion

• Refers to the degree of forward projection of femoral neck from the coronal plane of the femoral shaft.

• In an adult, it is about 10-15 degrees

APPLIED BIOMECHANICS

• The total hip component must withstand many years of cyclical loading equal to atleast 3 to 5 times the body weight and at time they may be subjected to overloads of as much as 10 to 12 times the body weight

• So, the basic knowledge of biomechanics of the THR and hip is necessary to properly perform the procedure, to successfully manage the problems that may arise during and after surgery, to select the components.

Head and nec k diameters :

• The neck with the smaller head tends to impinge on the edge of the cup during a shorter arc of motion which tends to loosen the components and dislocate the joint.

• The deep socket and beveled edges and the greater diameter of the head in comparison to the neck are the features that allow a greater range of motion.

Coefficient of friction and frictional torque :

• CE of friction is the measure of the resistance encountered in moving one object over the other.

• It depends on the material used, the finish of the surfaces ,temperature and the lubricant. – CE for normal joint- 0.008 to 0.02.– CF of metal on metal - 0.8 – CF of metal on HDPE (High density poly

ethylene) - 0.02

• A frictional torque force is produced when the loaded hip moves through an arc of motion. It is product of the frictional force times the length of the lever arm i.e., the distance of given point on the surface of the head moves during arc of motion.

• Frictional force depends on coefficient of friction, applied load and also on the surface area of contact between the head and socket.

• FT will increase with large size head.

• Theoretically it causes loosening of components.

WEAR :

Wear can be defined as the loss of material from the surfaces of the prosthesis as a result of motion between those surfaces. Material is lost in form of particulate debris.

Types :

Abrasive-THRAdhesive -THRFatigue - TKR

The factors that determine wear are : • CF of the substance and finishing surfaces • Boundary lubrication • Applied load • The sliding distance per each cycle• The hardness of the material • The number of cycles of movements The area of greater wear is in the superior

aspect of the socket where the body weight is applied to the femoral head.

• Wear is difficult to measures accurately, it may be measured by depth of penetration of the head with in the cup or the volume of debris produced or by a change in the weight of the polyethylene

• Newer methods- digitized x-rays and computer assisted wear measurements

• higher in younger and more active male patients. • Wear of more than 4 mm may result in neck

impingement on the edge of the cup and secondary loosening of the acetabulum.

INDICATIONS FOR THA :

• The primary indication for THA is incapacitating PAIN. Pain in the hip in the presence of destructive process as evidenced by X-ray changes is an indication.

• THA is an option for nearly all patients with diseases of the hip that cause chronic discomfort and significant functional impairment.

• Patients with limitation of movement, leg length inequality and limp but with little or no pain are not the candidates for THR.

• Most common reasons for total hip replacement:• Osteoarthritis 60 %• Rheumatoid arthritis 7 %• Fractures/dislocations 11 %• Aseptic bone necrosis 7 %• Revision 6 %• Other 9%

Common Causes of Hip Pain and Loss of Hip Mobility

Osteoarthritis • Usually occurs after age

50 and often in an individual with a family history of arthritis. In this form of the disease, the articular cartilage cushioning the bones of the hip wears away. The bones then rub against each other, causing hip pain and stiffness.

Causes (cont’d)

Rheumatoid Arthritis• a disease in which the

synovial membrane becomes inflamed, produces excessive synovial fluid, and damages the articular cartilage, leading to pain and stiffness.

Causes (cont’d)

Traumatic Arthritis • Can leads to a serious hip

injury or fracture. A hip fracture can cause a condition known as avascular necrosis. The articular cartilage becomes damaged and, over time, causes hip pain and stiffness.

Osteoarthritis Fracture

CONTRAINDICATIONS :

Absolute a) Patient with unstable medical illness that would significantly increase the risk of morbidity and mortality. b) Active infection of the hip joint or anywhere else in the body. Relative • Any process that is rapidly destroying bone eg. neuropathic joint, generalized progressive osteopenia.• Insufficiency of abductor musculature.• Progressive neurological disorder.

Hip Replacement Components

• Acetabular component - consists of two components– Cup - usually made of titanium– Liner - can be plastic, metal or

ceramic

• Femoral components Head Neck stem

FEMORAL COMPONENTS :• Neck length and offsets : The ideal femoral reconstruction reproduces

the normal center of rotation of femoral head, which can be determined by

-Vertical height (vertical offset) -Medial head stem offset ( horizontal offset) -Version of the femoral neck (anterior

offset)

• Vertical offset- LT to center of the femoral head. Restoration of this distance is essential in correction of leg length.

• Medial head stem offset- distance from the center of the femoral head to a line through the axis of the distal part of stem.

• Medial offset if inadequate, shortens the moment arm – limp, increase, bony impingement and dislocation.

• Excessive medial offset –increase stress on stem and cement which causes stress fracture or loosening.

• Version of the femoral neck : important in achieving stability of the prosthetic joint. The normal femur has 10-15 degree of anteversion.

CLASSIFICATION OF TOTAL HIP FEMORAL COMPONENTS :• Cemented : Charnely,Matche Brown,Muller ,alandruccio ,Aufranc –

Turner ,Sarmiento,Harris• Non cemented – Press Fit : Judet ,Lord ,Sivash , Porous Metal : Harris ,Galante,Hydroxyappatite coated

• Bipolar--Bateman ,Gilibertz ,Talwalkar

• Ceramic –Mittelmeir

• Polyacetate -Bombelli Mathes

• Custom made • Modular System

FEMORAL COMPONENTS USED WITH CEMENT

Cemented type

• Range of head sizes – 22, 26, 28 & 32 mm.• Incidence of dislocation is higher for smaller

head. • Neck diameter : Original charnleys was 12.5

mm but has been reduced to 10.5 mm – reduced neck diameter avoids impingement during flexion and abduction.

• Range of stem lengths -120 mm to 170 mm.• The main problem is mechanical loosening

and extensive bone loss associated with fragmented cement

CEMENTLESS STEMS WITH POROUS SURFACES

Basic principle

• Based on the principle-bone ingrowth from the viable host bone into porous metal surfaces of implant.

• Indications for cementless components involves 1.primarily active young patients

2.and revisions of failed cemented components.

• Two prerequisites for bone ingrowth 1.immediate implant stability at the time of surgery 2.and intimate contact between the porous surface and

viable host bone• Implants must be designed to fit the endosteal cavity

of the proximal femur as closely as possible. • In general, the selection of implant type and size, as

well as the surgical technique and instrumentation, must all be more precise than with their cemented counterparts

Porous Coated Implants

Current porous stem designs

• 1.titanium alloy with a porous surface of commercially pure titanium fiber-mesh or beads

• and (2) cobalt-chromium alloy with a sintered beaded surface.

• 2 shapes- Cementless total hip stems are of two basic shapes: straight and anatomical

• The aim of both types is to provide optimal fit both proximally and distally and thereby achieve axial and rotational stability by virtue of their shape

Types of porous coated stems• Circumferential porous coating-first

generation femoral stems• Extensive coated stems• Proximally coated stem – twice the incidence

of thigh pain(stem tip abutment on the anterior cortex of femur)

• Tapered femoral stems• Stems with hydroxyapetite coatings

NON POROUS CEMENTLESS FEMORAL COMPONENTS• nonorous femoral

implants have surface roughening that provide a macrointerlock with bone

• No capacity for bone ingrowth but provides lasting implant stability

• With the concerns about fatigue strength, ion release and adverse femoral remodeling, these non porous stems came into use over porous stems

Advantages of cementless femoral stem prosthesis

• No cement required and problem related to cement to bone and cement implant interface reduced

• In young active patients• Decreased incidence of asceptic loosening• Less bone destruction• Circumferential porous coating of proximal stem

provide effective barrier to ingress debris particle and thus limit early development of osteolysis of distal stem

ACETABULAR COMPONENTS :• The articulating surface of all acetabular

components is made of UHMWPE. Most systems feature a metal shell with an outside diameter of 40 to 75 mm which is mated to a polythene liner.

• optimum position for the prosthetic socket which should be inclined 45⁰ or less to maximize stability of the joint.(normal 55⁰)

Types : • Cemented acetabular components. • Cementless acetabular components. • Custom made acetabular components

CEMENTED ACETABULAR COMPONENTS

• Original sockets- thick walled polyethylene cups. Vertical and horizontal grooves on external surface to increase stability within the cement mantle

• wire markers were embedded in plastic to allow better assessment of position on postoperative roentgenograms.

• More recent designs have a textured metal back which improves adhesion at the prosthesis cemented interface. A flange at the rim improves pressurization of the cement.

• used in elderly patients, tumour reconstruction and the circumstances with less chances of bony ingrowth as in revision THR.

Cementless Acetabular Components

• Most cementless acetabular components are porous coated over their entire circumference for bone ingrowth

• Fixation of the porous shell with transacetabular screws

• Pegs and spikes driven into prepared recesses in the bone provide some rotational stability but less than that obtained with screws.

• ZTT socket Hemispherical , porous

coated cup designed with dome screw holes and transacetabular screws for stability. Six peripheral screw holes provide choice of screw locations for additional stability and also lock in the polyethylene insert.

Two techniques involved 1.Initial stability of the metal shell against the

acetabular bone using screws, spikes , lugs, or fins2. Stratch fit- underream the acetabular bone bed by

1-2 mm and use the roughness of the outer surface of metal shell to achieve scratch fit

• Expansion cup method-Cup diameter is reduced with with a special instrument , cup then implanted and then allowed to return to initial diameter.

polyethylene liner

• Most modern modular acetabular components are supplied with a variety of polyethylene liner choices

• The polyethylene liner must be fastened securely to the metal shell.

• Current mechanisms include plastic flanges and metal wire rings that lock behind elevations or ridges in the metal shell, and peripherally placed screws

• in vivo dissociation of polyethylene liners from their metal backings has been reported micromotion between the nonarticulating side of the liner and the interior of the shell may be a source of polyethylene debris generation, or “backside wear.”

Alternative Bearings

• Osteolysis secondary to polyethylene particulate debris has emerged as the most notable factor endangering the long-term survivorship of total hip replacements.

• alternative bearings have been advocated to diminish this problem

• These are- -highly cross linked polyethylene -metal-on-metal -ceramic-on-ceramic -Ceramic on Polyethylene

Highly Cross-Linked Polyethylene • Higher doses of radiation(gamma or

electron,10mrad) can produce polyethylene with a more highly cross-linked molecular structure.

• This material has shown remarkable wear resistance.

• Only short-term data on the performance of highly cross-linked polyethylenes are presently available

• Diadvantage -lower fracture toughness and tensile strength

Metal-on-Metal Bearings • Metal-on-metal implants seem to be tolerant of

high impact loading, and mechanical failure has not been reported.

• wear rates less than 10 mm/y for modern metal-on-metal articulations

• But there remains major concern regarding the production of cobalt and chromium metallic debris, and its elimination from the body.

• metal-on-metal (MOM) bearings have a ‘suction-fit’ less chance of dislocation

(J Bone Joint Surg [Br] 2003;85-B:650-4)

Ceramic-on-Ceramic Bearings• Alumina ceramic has many properties that make it

desirable as a bearing surface in hip arthroplasty• high density- surface finish smoother than metal

implants• The hydrophilic nature- ceramic promotes lubrication• Ceramic is harder than metal and more resistant to

scratching from third-body wear particles.• The linear wear rate of alumina-on-alumina has been

shown to be 4000 times less than cobalt-chrome alloy–on–polyethylene.

• Ceramic-on-ceramic arthroplasties may be more sensitive to implant malposition than other bearings. (J Bone Joint Surg [Br] 2003;85-B:650-4

ROENTEGENOGRAPHIC EVAL U ATION• AP view of pelvis with both hips with upper third

femur with limbs in 15degrees internal rotation. • Spine, knee x-ray taken Note the following : • Acetabulum : Bone stock, floor, migration,

protrusio, osteophytes and cup size. • Femur : Medullary cavity (size & shape). Limb length discrepancy Neck.

OperationRemoving the Femoral Head

• Once the hip joint is entered, the femoral head is dislocated from the acetabulum.

• Then the femoral head is removed by cutting through the femoral neck with a power saw.

Reaming the Acetabulum

• After the femoral head is removed, the cartilage is removed from the acetabulum using a power drill and a special reamer.

• The reamer forms the bone in a hemispherical shape to exactly fit the metal shell of the acetabular component.

Inserting the Acetabular Component

• A trial component, which is an exact duplicate of your hip prosthesis, is used to ensure that the joint will be the right size and fit for the client.

• Once the right size and shape is determined for the acetabulum, the acetabular component is inserted into place.

Preparing the Femoral Canal

• To begin replacing the femoral head, special rasps are used to shape and scrape out femur to the exact shape of the metal stem of the femoral component.

• Once again, a trial component is used to ensure the correct size and shape. The surgeon will also test the movement of the hip joint.

Inserting Femoral Stem

• Once the size and shape of the canal exactly fit the femoral component, the stem is inserted into the femoral canal.

Attaching the Femoral Head

• The metal ball that replaces the femoral head is attached to the femoral stem.

The Completed Hip Replacement

• Client now has a new weight bearing surface to replace the affected hip.

• Before the incision is closed, an x-ray is made to ensure new prosthesis is in the correct position.

COMPLICATIONS :• Inherent to any major surgical procedure

in elderly patients. • Specifically related to the procedure of

THR:

EARLY

Nerve injury Hemarthrosis/vascular injury Thromboembolism Bladder injuries

INDEPENDENT OF TIME

Infection Dislocation Trochanteric non union Femoral fracture Limbs length discrepancy

LATE

-Loosening-Component failure -Osteolysis -Heterotrophic ossification

Dislocation or subluxation :• Can occur in 3 % Causes : • Excess retroversion or

ateversion• Small size head,• Laxity of the soft tissue

around the joint.• Insufficient offset. Treatment : Reduction by : Bigelows or

Stimsons method

Heterotopic ossification :• Most commonly develops

in male patients who have been operated for anklyosing spondylitis

• Cause is unknown • Loss of motion is the

predominant functional limitation

Management : • Prophylaxis: Diphosphates • Low dose NSAIDs,

indomethacin 75mg/day x 6 weeks

• Radiotherapy

9. Loosening :• Femoral and acetabular loosening are the most serious

femoral and acetabular long-term complications.• Most common indications for revision arthroplasty. Cemented femoral loosening : • Loosening of a femoral stem as defined as radiographically

demonstrable change in the mechanical integrity of the load carrying cemented femoral component.

• Loosening is present if a radiolucent zone more than 2 mm wide is seen. Especially if noted about the entire cement mass and if it is increased progressively in width.

BIOMECHANICAL CONSIDERATIONS IN THR : • Lengths of the lever arm can are surgically

changed to approach r ratio of 1:1 (which reduces the hip total load by 30 % ).

• Abductor lever arm can be increased either by increasing the medial offset of the femoral component or lateral / distal reattachment of greater trochanter.

• Joint reaction forces are minimal if hip center is placed in anatomical position.

• Adjustment of neck length is important as it has effect on both medial offset and vertical offset. Neck length typically ranges from 25 to 50 mm.

• Femoral components must be produced with a fixed neck shaft angle typically about 1350.

• Restoration of the neck in coronal plane Increased anteversion – anterior dislocation Increased Retroversion – posterior dislocation • Socket depth and beveled edges and greater

diameter of head in comparison of neck allow greater range of motion.

•Neck diameter should approach that to make neck stronger especially with small femoral heads. •Frictional torque of small head will be less compared to larger head. •Increasing stem length and cross sectional area increases the stress in the stem. •Any loading of proximal medial neck likely to decrease bony resorption and reduces stresses on cement. •Loose fitted stem – increase stresses in proximal femur.

Cementless femoral stem : • Fixation by bone ingrowth is defined as an implant with

minimal or no opaque line formation around the stem. • An implant is considered to have a stable fibrous

ingrowth when no progressive migration occurs but an extensive radio-opaque line forms around the stem. These lines surround the stem in parallel fashion and are separated from the stem by a radiolucent space upto 1 mm wide.

• An unstable implant is defined as one with definitive evidence of either progressive migration within the canal and is atleast partially surrounded by divergent radio-opaque lines that are more widely separated from the stem at its extremities.

Acetabular loosening : • In general it is agreed that the acetabular cup is

loose if a radiolucency of 2 mm or more in width is present in all three zones.

• “The diagnosis of loosening is accepted in most instances if the radiolucent zone about one or both components is 2mm or more in width and the patient has symptoms on weight bearing and motion that are relieved by rest”.

• Solution is the revision THR

Depuy Pinnacle

ZIMMER DUROM

Resurfacing Arthroplasty

• Surface hip replacement consists of resurfacing the acetabulum with a thin layer of bearing surface, and replacement of only the femoral head (not neck) with a metal ball.

• The ideal candidate for a resurfacing hip arthroplasty is a young (<60 years old), active individual, with normal proximal femoral anatomy and bone density who might be anticipated to outlive a conventional hip arthroplasty.

• The procedure is more technically demanding than conventional hip arthroplasty, particularly with reference to exposure of the acetabulum because the femoral head is not resected.

• Although the procedure is conservative of bone, a more extensile soft-tissue dissection is required for adequate exposure. Resurfacing of the femoral head alone as a hemiarthroplasty may be valuable in young patients with osteonecrosis.

Thank You