Implant therapy has got advanced over these years and has become a reliable treatment for the replacement of damaged parts. Implant therapy varies based on the type of tissue taken into consideration. There are basically two types of tissue implants, hard and soft tissue implants. This blog is all about hard tissue implants.

There are about 206 bones in the human body in which the longest bone is the femur and the smallest bone is the stapes.

Hard tissues are nothing but tissues that are mineralized and have a hard/firm intercellular matrix. They are also called calcified tissues. The hard tissues include bone, teeth, enamel, dentin, and cementum. The breakage of these bones leads to bone fracture that needs to be fixed. The fixation methods are of three types:

  1. Simple fixation
    • Here the injury is simple and it is healed with the help of a splint or cast.
  2. External fixation devices
    • External fixators are used in severe traumatic injuries.
    • External fixation allows easy access to the wounds.
    • This is used in cases where the use of internal fixation is prevented (like illness).
  3. Internal fixation devices
    • This method is used in cases of bad injury to the bone.
    • This involves the fixation of internal fixators to facilitate healing and repairing of the bone.
    • The different internal fixation tools used are
      • Screws:-
        • Used to provide fixation or in conjunction with fracture plates
        • Self-tapping and non-self-tapping screws are some types
        • Holding power and pull-out strength of both types are the same
      • Plates:-
        • Use to provide stabilization for the broken bone.
        • Used along with screws.
        • Over-tight fixation of the plate with the screws result in necrosis.
        • Bone plates must be strong enough to bear and support the load that is typically placed on the bone.
        • To overcome the bending movement, restriction of movement is essential in the early stages of healing.
        • The materials used for the plate are stainless steel, cobalt-based alloys, titanium, polylactic acid, polycaprolactone, composites, etc.,
      • Wires and pins:-
        • Used to hold the fragments of the bone together.
        • Steinman pin is commonly used.
        • Three types of pin tips are diamond, trochar, and cove, among which trochar tip is efficient in cutting and useful for cortical bone insertion.
        • The materials used are stainless steel and titanium.
      • Intramedullary (IM) Nails:-
        • IM rods are used to align and stabilize fractures.
        • These rods are inserted into the bone marrow in the center of the long bones.
        • Earliest IM nails were “triangular” or “V” shaped. Now they are in stable rotational clover leaf shape.
        • Titanium is preferred over stainless steel.

Spinal fixation devices

Spinal fixation is also known as vertebral fixation. Here two or more vertebrae are anchored to each other through the spinal fixation device to reduce vertebral mobility and avoid damage to the spinal cord. The device is usually made of titanium-based permanent rigid or semi-rigid prosthesis. A bio-resorbable material can be used as a less common alternative.

The six major joint replacements are explained as follows:

  1. Knee replacement implants
  2. Hip replacement implants
  3. Shoulder replacement implants
  4. Ankle replacement implants
  5. Elbow joint implants
  6. Wrist and finger joint implants

Knee replacement implants

The implants here include the patellar component, femoral component, tibial component and plastic spacer. The various types of knee replacement are total knee replacements, partial/unicompartmental knee replacement, kneecap replacement, and complex or revision knee replacement. Hydroxyapatite is used in porous-coated knee implants. This is a bioactive agent that will bond as the bone grows into it. The knee replacement materials include:

  • Titanium and its alloys
  • Cobalt chromium alloys
  • Polyethylene
  • Tantalum
  • Zirconium
Knee replacement implant

Learn more about knee implants

Hip replacement implants

The hip implants include components such as an acetabular cup, polyethylene liner, femoral head, neck, and stem. The prosthetic devices can be porous coated or nonporous coated. Porous coated is a better fit. The implant fixations can be cemented or cementless. Preferably cemented is chosen for elder people and cementless for younger ones. The materials chosen for the implants may be metal and plastic implants, metal on metal implants, or ceramic on ceramic implants. The materials are listed below:

  • Titanium
  • Stainless steel
  • Co-Cr
  • Polyethylene
  • Oxidized zirconium (for ceramics)
Hip replacement implant

Learn more about hip implants

Shoulder replacement implants

The implant of shoulder replacement includes a metal ball with a stem and a plastic socket. Here either the replacement of the head of the humerus bone(i.e., ball) or the replacement of both the ball and socket is done. Similar to types of knee replacement, shoulder replacement is also classified into four types such as total shoulder replacement, stemmed hemiarthroplasty, reverse total shoulder replacement, and resurfacing hemiarthroplasty.

Ankle replacement implants

The ankle implants include tibial and talar components along with the plastic spacer. The tibial component replaces the socket portion of the ankle and it is made up of two parts: a metal tray(a flat metal piece) and a plastic cup. The talar component replaces the top of the talus. The implant materials used are Co-Cr alloy, UHMWPE, and carbon-fiber-reinforced UHMWPE (Ultra-high-molecular-weight polyethylene).

Elbow joint replacement implants

The elbow joint implant includes humeral and ulnar components along with the hinge pin. The ulnar component is a shorter metal stem whereas the humeral component is a longer stem. The hinge allows for elbow movement. The types of elbow joint implants are linked and unlinked.

Elbow joint replacement implant

Wrist and finger joint replacement implants

The wrist joint implant includes radial and distal components where the radial component is the flat metal piece along with the stem and the distal component is the metal. A plastic cup fits onto the metal piece, forming a socket for the artificial wrist joint. Finger joint replacements are successful but the major failure here is the mobility. The types of finger joint implants are resectional arthroplasty, hinge type, polycentric hinge type, dynamic spacer, and hemisection arthroplasty.

The major causes of joint replacement failure are,

  • Osteolysis
  • Infection
  • Instability
  • Implant failure
  • Mechanical loosening
  • Peri-prosthetic failure

So to overcome these failures, the material chosen must satisfy all the conditions including biocompatibility, osseointegration, etc., and the proper medical replacement surgery has to be performed to avoid such failures.



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