Joint Replacements Hip Natural Joint Single Component Replacement

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Joint Replacements • Hip - Natural Joint - Single Component Replacement - Double Cup

Joint Replacements • Hip - Natural Joint - Single Component Replacement - Double Cup Replacement - Total Hip Arthroplasty (THA) Articulating Surface Fixation Press-Fit Implants Active Mechanical Fixation Bone Cement Porous Ingrowth Cement with Ingrowth Direct Chemical Bonding Stress Shielding • Knee - Knee Arthroplasty - Non-hinged Design - Knee Replacement Organi Artificiali e Protesi Prof. O. Sbaizero • • • Ankle Shoulder Elbow Wrist Finger

Joint Replacements Used as a last resort if the natural joint is unsalvageable Goals:

Joint Replacements Used as a last resort if the natural joint is unsalvageable Goals: Maintenance or return of function Mechanical stability Reduction of pain Ease of installation/surgical procedure Constraints: Capable of withstanding physiologically applied loads Minimize wear Minimize corrosion and leaching Minimize infection (which can lead to failure) Minimize stress shielding Organi Artificiali e Protesi Prof. O. Sbaizero

Hip - Natural Joint (1) • Ball-and-socket articulation • 6 degrees of freedom •

Hip - Natural Joint (1) • Ball-and-socket articulation • 6 degrees of freedom • Movement limited by bony surfaces and musculature • Minimal joint friction due to cartilaginous surfaces • Load distributed over larger area due to deformation of viscous cartilage • Experiences forces greater than 7 x body weight during normal activity - Due in a large part to muscle forces • See Figure 1 Organi Artificiali e Protesi Prof. O. Sbaizero

Figure 1 Anatomy and cartilage contact surface of the hip. Organi Artificiali e Protesi

Figure 1 Anatomy and cartilage contact surface of the hip. Organi Artificiali e Protesi Prof. O. Sbaizero

Hip - Natural Joint (2) • General properties of implant: - Maintain 6 degrees

Hip - Natural Joint (2) • General properties of implant: - Maintain 6 degrees of freedom - Ball-and-socket articulation to simulate normal joint - Must be well fixed within remaining bone to prevent loosening - Loosening is a major cause of implant failure - Normal muscle and ligament attachments should be maintained to the greatest extent possible to provide normal function and stability • Dealing with two articulating surfaces - Can be considered separately or as a unit Organi Artificiali e Protesi Prof. O. Sbaizero

Hip - Single Component Replacement • Early method of joint repair • Either an

Hip - Single Component Replacement • Early method of joint repair • Either an acetabular or femoral head component is used but not both • Damage to acetabulum: - Polymeric acetabular cup inserted into the surgically prepared site - Natural femur articulates against polymeric surface • Damage to femoral head: - Alloy or polymeric cup placed over the natural femoral head, providing a new surface - Articulates against natural acetabulum, which may be resected to fit the cup Organi Artificiali e Protesi Prof. O. Sbaizero

Hip - Double Cup Replacement Both the femoral cup and the acetabular cup are

Hip - Double Cup Replacement Both the femoral cup and the acetabular cup are used Dead tissue is removed from the acetabulum before insertion of the cup See Figure 2 Usually use two different materials - why? In order to reduce the friction. # Two like materials are likely to have a higher coefficient of friction due to the chemical bonding of the surfaces. Benefits: - Removes less tissue than other designs (such as THA), which makes revision easier - Provides a near-normal stress-distribution Problems: - Can only be used for treatment of degenerative joint disease (ie. osteoarthritis or rheumatoid arthritis), not for the repair of fractures Organi Artificiali e Protesi Prof. O. Sbaizero

Figure 2 Drawing of a double cup arthroplasty Organi Artificiali e Protesi Prof. O.

Figure 2 Drawing of a double cup arthroplasty Organi Artificiali e Protesi Prof. O. Sbaizero

Hip - Total Hip Arthroplasty (THA) • Uses an acetabular component in conjunction with

Hip - Total Hip Arthroplasty (THA) • Uses an acetabular component in conjunction with a femoral hip stem • Diseased or fractured femoral head/neck is removed and the intermedullary canal is drilled and reamed to prepare it for the hip stem - May damage intramedullary blood supply of remaining bone • Both the acetabular cup and the femoral stem must be adequately fixed • Most common form of hip prosthesis Benefits: - Can be used to repair femoral fractures - Any diseased tissue is completely removed Problems: - Changes the stress distribution in the bone - Adequate fixation is difficult due to the bending moment on the femoral neck - Revision is difficult due to the amount of tissue removed in the procedure - Matching the geometry of the original femur is difficult - Intramedullary blood supply may be damaged Organi Artificiali e Protesi Prof. O. Sbaizero

THA Designs - Articulating Surface 1. Ball-and-socket - uses a smaller ball than the

THA Designs - Articulating Surface 1. Ball-and-socket - uses a smaller ball than the orignal joint, articulating against an acetabular cup 2. Trunion - only femoral stem used, while natural acetabular articulation is maintained; uses a ball size more closely approximating that of the natural joint (ball = 2 x that of ball-and-socket) 3. Retained ball-and-socket - uses small ball held within a constrained joint surface; increases stability; more difficult to implant as unit usually comes pre-assembled See Figure 4 Organi Artificiali e Protesi Prof. O. Sbaizero

Figure 4 Various designs for total hip arthroplasty. From left: Ball and Socket, Double

Figure 4 Various designs for total hip arthroplasty. From left: Ball and Socket, Double Cup, Trunion, Retained Ball and Socket Organi Artificiali e Protesi Prof. O. Sbaizero

THA Designs - Fixation Both the femoral and acetabular components must be firmly fixed

THA Designs - Fixation Both the femoral and acetabular components must be firmly fixed Concerns: - Implant lies in cancellous bone - Stress concentrations at points of sharp contact make the bone more necrotic, resulting in potential loss - Stress-shielding due to poor load transfer results in bone loss - Significant micromotion will result in capsule formation and lack of permanent joint fixation - Some amount of micromotion will cause damage to "fixation layer" (natural or man-made) resulting in further loosening - Loosening is the primary cause of failure among joint replacements Possible means of fixation (See Figure 5): - Passive mechanical fixation - Active mechanical fixation - Bone cement - Porous ingrowth - biological fixation - Direct chemical bonding of surfaces - Combination of bone cement and porous ingrowth Organi Artificiali e Protesi Prof. O. Sbaizero

Figure 5 Various methods of hip stem fixation. Organi Artificiali e Protesi Prof. O.

Figure 5 Various methods of hip stem fixation. Organi Artificiali e Protesi Prof. O. Sbaizero

THA Fixation - Press-Fit Implants • Snug fit is obtained between the implant and

THA Fixation - Press-Fit Implants • Snug fit is obtained between the implant and the surrounding bone • Some micromotion must be expected and a fibrous capsule will be formed • Stress transfer is through compressive forces only • Works in hip implants due to wedge shape of implant and compressive loading situation • Requires surgical precision - no back-up of filling excess space with another material • Implant may continue to sink under a life-time of normal loading • Used mostly on implants with larger stem sizes Benefits Easier revision No added toxic effects from fixation material Problems Stress transfer not optimal -- only through proximal end of implant Any micromotion cause a positive feedback cycle leading to failure: Motion -- fibrous capsule formation and tissue damage -- motion -- loosening -- motion Organi Artificiali e Protesi Prof. O. Sbaizero

THA Fixation - Active Mechanical Fixation Uses screws, pins, and wires Can also include

THA Fixation - Active Mechanical Fixation Uses screws, pins, and wires Can also include use of bone cement (to be discussed separately) Primarily used for attachment of acetabular component Benefits: - Provides good immediate fixation, either as a temporary means until ingrowth occurs or as permanent fixation Problems: - Stress concentrations at any screw insertions - Corrosion due to additional metal in design - Metal to metal contact which can increase corrosion and wear Organi Artificiali e Protesi Prof. O. Sbaizero

THA Fixation - Bone Cement (1) • PMMA is mixed into a polymer during

THA Fixation - Bone Cement (1) • PMMA is mixed into a polymer during surgery and forced into the prepared cavity • Sets quickly and holds the implant in place • Two interface surfaces: Equal rates of failure - Implant/cement # Can be improved by precoating the metal with bone cement or PMMA polymer during manufacturing so that the new bone cement may adhere more readily - Cement/bone # Inherent problems due to nature of bone and cement # Toxicity of material # Weakness of cement # UTS of 30 MPa vs. 60 -130 MPa for compact bone and 660 -1800 MPa for alloys # Porosity in PMMA further reduces strength Organi Artificiali e Protesi Prof. O. Sbaizero

THA Fixation - Bone Cement (2) Benefits: - PMMA is viscoelastic and so acts

THA Fixation - Bone Cement (2) Benefits: - PMMA is viscoelastic and so acts in some ways as a shock absorber - Distributes the load more evenly and reduces stress concentrations and stress shielding compared to a press-fit design Problems - Monomer may be toxic and can leach from incompletely polymerized cement - Polymerization is an exothermic process and can damage tissue - Two interfaces exist which must each participate in providing fixation - Cement may deteriorate and either - Release particles into the surrounding tissue which can aggrevate wear or irritiate tissue - See a reduction in the mechanical properties Organi Artificiali e Protesi Prof. O. Sbaizero

THA Fixation - Porous Ingrowth (1) • Development of a permanent and viable interface

THA Fixation - Porous Ingrowth (1) • Development of a permanent and viable interface between the bone and implant • Will occur in almost any inert material given the following conditions: - Adequate pore size - greater than about 150 µm - Pores are contiguous with the surface of the implant - Pores are in contact with the remaining bone tissue - Ingrown bone should be subject to normal loading to prevent resorption Benefits: - Permanent bond which should not deteriorate in the normal physiological environment - Coating area can be changed to vary the stress transfer characteristics Problems - Surgically unforgiving, requires precision - Requires long immobilization to allow for initial ingrowth - Porous coating can act as a dead space and provide a breeding ground for any infection which may be present - Damage to the porous coating or the interface between the coating and implant cannot be repaired Organi Artificiali e Protesi Prof. O. Sbaizero

THA Fixation - Porous Ingrowth (2) Design modifications: - Pre-coating metals with ceramics or

THA Fixation - Porous Ingrowth (2) Design modifications: - Pre-coating metals with ceramics or carbons # Reduces corrosion # Bioactive ceramics may help induce bone ingrowth - Coating metal stem with porous polymer # Reduces corrosion # More gradually transfers stress from metal to bone due to low modulus # Helps prevent stress-shielding # Weak interface between polymer and metal may fail under loading Organi Artificiali e Protesi Prof. O. Sbaizero

THA Fixation - Cement with Ingrowth • Cement used for initial fixation while providing

THA Fixation - Cement with Ingrowth • Cement used for initial fixation while providing space for later tissue ingrowth • A porous cement alone is not desirable due to the reduced strength • Consists of incorporating a resorbable particle within the bone cement - Inorganic bone - Calcium phosphates • Theory: Strength of interface will convert from that of a cemented system to an ingrown system (See Figure 6) • Shown in dogs that at 5 months the composite, resorbable cement provided an increased shear strength between the implant and bone compared to the cement alone (See Figure 7) • 30 percent bone particles provide sufficient "porosity" while maintaining reasonable values for other properties • Not yet demonstrated in humans Organi Artificiali e Protesi Prof. O. Sbaizero

Figure 6 Theoretical variation in fixation strength due to resorbable cement and bone ingrowth.

Figure 6 Theoretical variation in fixation strength due to resorbable cement and bone ingrowth. Organi Artificiali e Protesi Prof. O. Sbaizero

Figure 7 Difference in variation of fixation strength in cemented stems with and without

Figure 7 Difference in variation of fixation strength in cemented stems with and without resorbable particles within the cement. Based on implantation study in dogs. Organi Artificiali e Protesi Prof. O. Sbaizero

THA Fixation - Other Direct Chemical Bonding - Using glass-ceramic coating on a metallic

THA Fixation - Other Direct Chemical Bonding - Using glass-ceramic coating on a metallic surface - Theoretical only - not yet shown to be feasible in application Organi Artificiali e Protesi Prof. O. Sbaizero

THA Design - Stress Shielding (1) • Characteristics beyond the obvious of reducing implant

THA Design - Stress Shielding (1) • Characteristics beyond the obvious of reducing implant stiffness • Generally, the stresses in the bone are reduced due to the inclusion of an implant (See Figure 8) - Exception: the distal end of the implant, where stresses are increased • Typical designs and basic stress transfer examples 1. Traditional cemented without lip (See Figure 9) - Force transmitted down length of medullary canal - Reduced stresses along length of implant - Increased stresses at distal end of implant due to bending moment about this point Organi Artificiali e Protesi Prof. O. Sbaizero

Figure 8 Stresses on the surface of a femoral stem with and without an

Figure 8 Stresses on the surface of a femoral stem with and without an implant in place. Organi Artificiali e Protesi Prof. O. Sbaizero

Figure 9 Typical design of a hip implant without a proximal lip. Organi Artificiali

Figure 9 Typical design of a hip implant without a proximal lip. Organi Artificiali e Protesi Prof. O. Sbaizero

THA Design - Stress Shielding (2) 2. Traditional cemented or press-fit with lip (See

THA Design - Stress Shielding (2) 2. Traditional cemented or press-fit with lip (See Figure 10) - Lip at proximal end of stem acts to transfer load to cortical bone in the femoral neck region - Stresses still reduced due to inclusion of stiff implant, but to a lesser degree - Load transferred through cortex as well as down medullary canal - Trabecular bone still shielded from stresses - will resorb 3. Porous coated (See Figure 11) A. Fully porous coated: # Load transmitted gradually down medullary canal # More load borne by proximal region due to ingrowth, however greatest amount of load transfer is in distal region of implant B. Partially porous coated: # Only top portion of implant coated # Load transferred gradually to trabecular bone then distributed down length of bone # More closely approximates true loading situation Organi Artificiali e Protesi Prof. O. Sbaizero

Figure 10 Typical design of a press-fit hip implant with a proximal lip Typical

Figure 10 Typical design of a press-fit hip implant with a proximal lip Typical design of a cemented hip stem with a proximal lip. Organi Artificiali e Protesi Prof. O. Sbaizero

Figure 11 Typical design of a hip implant with full porous coating layer Typical

Figure 11 Typical design of a hip implant with full porous coating layer Typical design of a hip stem with porous coating limited to the proximal region. Organi Artificiali e Protesi Prof. O. Sbaizero

Joint Replacements - Knee • Natural Joint (See Figure 12)- - Hinge joint -

Joint Replacements - Knee • Natural Joint (See Figure 12)- - Hinge joint - 1 degree of freedom - flexion and extension - 3 articulating surfaces Femur Tibia Patella - More complicated geometry than hip - Less stable than hip, especially with damage or removal of natural ligament connections - Center of rotation not constant • Both hinged and non-hinged joints proposed (See Figure 13) • Two components plus a patellar "button" if patella removed - Femoral and tibial components • Combination of alloy and polymer Organi Artificiali e Protesi Prof. O. Sbaizero

Figure 12 Anatomy of the knee. Organi Artificiali e Protesi Prof. O. Sbaizero

Figure 12 Anatomy of the knee. Organi Artificiali e Protesi Prof. O. Sbaizero

Figure 13 Designs of various knee implants. Organi Artificiali e Protesi Prof. O. Sbaizero

Figure 13 Designs of various knee implants. Organi Artificiali e Protesi Prof. O. Sbaizero

Knee Arthroplasty - Non-hinged Design • Metallic femoral component which is curved like the

Knee Arthroplasty - Non-hinged Design • Metallic femoral component which is curved like the natural femoral condyles - May be cemented, porous coated, or screwed to maintain fixation - Slides against tibial articulation to imitate natural motion • Metallic tibial plate which transfers stress both to the trabecular and cortical bone - Extending the plate to the cortical surfaces prevents sinkage in the trabecular bone due to loading - May be porous coated, cemented, or screwed to maintain fixation • Polymeric tibial articulating surface - Reduces friction - Subject to significant wear - Snaps directly to metallic tibial tray Organi Artificiali e Protesi Prof. O. Sbaizero

Knee Replacement • Surgical area must be completely cleaned of bone cement and bone

Knee Replacement • Surgical area must be completely cleaned of bone cement and bone debris to prevent aggravation of wear processes • Porous coatings should be used primarily in patients with healthy bone tissue due to the reliance on ingrown tissue for stability • Hemi-arthroplasties can also be performed where only one side of femoral condyles is replaced with an implant • Cruciate ligaments may or may not be left intact Organi Artificiali e Protesi Prof. O. Sbaizero

Joint Replacements - Ankle • Difficult joint to replace • Consists of three articulating

Joint Replacements - Ankle • Difficult joint to replace • Consists of three articulating surfaces • Acts as a universal joint, with limited rotation as well as dorsiflexion and plantarflexion • Joint responds in a gliding motion, instead of as a hinge like the knee • Incongruent (non-matching surfaces) and congruent replacements (See Figure 14) - Incongruent joints are less stable and experience higher stress concentrations than congruent designs - Congruent joints have superior functional properties due to greater contact surface area • Typically constructed of UHMWPE and Co-Cr alloys • Used only for a short time • Problems with pain, loosening, and limited motion Organi Artificiali e Protesi Prof. O. Sbaizero

Figure 14 (1) Organi Artificiali e Protesi Prof. O. Sbaizero

Figure 14 (1) Organi Artificiali e Protesi Prof. O. Sbaizero

Figure 14 (2) Organi Artificiali e Protesi Prof. O. Sbaizero

Figure 14 (2) Organi Artificiali e Protesi Prof. O. Sbaizero

Joint Replacement - Shoulder • Shoulder - Ball-and-socket articulation with largest amount of motion

Joint Replacement - Shoulder • Shoulder - Ball-and-socket articulation with largest amount of motion in body • Less constrained by muscles and bones than hip (See Figure 15) • Unstable joint due to lack of bony containment • The natural joint is incongruent - the radius of curvature of the two surfaces does not match - Allows some translation of the humeral component (arm) within the glenoid (shoulder component - part of scapula or shoulder blade) • Current debate over whether the implants should be congruent to provide stability (and possibly develop high stress concentrations and wear) or incongruent to retain maximum motion • General ball-and-socket type of implant design (See Figure 16) • Can be fixed in the same way as hip replacements Organi Artificiali e Protesi Prof. O. Sbaizero

Figure 15 Anatomy of the shoulder. Organi Artificiali e Protesi Prof. O. Sbaizero

Figure 15 Anatomy of the shoulder. Organi Artificiali e Protesi Prof. O. Sbaizero

Figure 16 Three designs of shoulder implants. Organi Artificiali e Protesi Prof. O. Sbaizero

Figure 16 Three designs of shoulder implants. Organi Artificiali e Protesi Prof. O. Sbaizero

Joint Replacement - Elbow • Hinge-type joint allowing flexion and extension, but with a

Joint Replacement - Elbow • Hinge-type joint allowing flexion and extension, but with a polycentric motion • Most implants are either hinge joints (either loose or rigid) or surface replacements • Major complications are loosening and infection, due to the small amount of soft tissue coverage at this location Organi Artificiali e Protesi Prof. O. Sbaizero

Joint Replacement - Wrist • Multiple bone articulations allowing flexion, extension, adduction and abduction

Joint Replacement - Wrist • Multiple bone articulations allowing flexion, extension, adduction and abduction • Both ball-and-socket and "space-filler" type joints have been developed (See Figure 17) - Ball-and-socket types are alloy and polymer systems like the hip - Space-filler joints are made of silicon rubber and provide a flexible joint instead of one that articulates • Difficult to place prosthesis in correct position due to removal of one of the carpal bones (capitate) during surgery • The unnatural position of the implant results in constraing of movement and excessive beding moments in adduction or abduction • Poor clinical results due to complex motion and surgical inconsistency Organi Artificiali e Protesi Prof. O. Sbaizero

Figure 17 Various designs of wrist implants. Organi Artificiali e Protesi Prof. O. Sbaizero

Figure 17 Various designs of wrist implants. Organi Artificiali e Protesi Prof. O. Sbaizero

Joint Replacement - Finger Each finger has three independent joints (thumb has two) Implant

Joint Replacement - Finger Each finger has three independent joints (thumb has two) Implant designs (See Figure 18): 1. Resectional arthroplasty - damaged articulating surfaces simply removed Reduces pain, but joint is unstable; Limited motion 2. Hinge Excessive wear and breakage 3. Polycentric Unstable; Good duplication of anatomical motion 4. Space-filler Good clinical success; Poor grip strength Constucted of silicon rubber, composites, or polypropylene • Implant is fixed through the fibrous capsule that forms - Important in fingers as cortical bone in the phlanges is too thin to withstand concentrated loads from cement fixation or bony ingrowth Organi Artificiali e Protesi Prof. O. Sbaizero

Figure 18 Various designs of finger implants. Organi Artificiali e Protesi Prof. O. Sbaizero

Figure 18 Various designs of finger implants. Organi Artificiali e Protesi Prof. O. Sbaizero