Cartilage Repair Biology

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  I. Biologic injections are indicated for focal and diffuse chondral disease of the hip.

  
  
  
  
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     Hyaluronic acid (HA): intra-articular injection of HA reduces pain and inflammation. ¹ HA binds to receptors on the cluster determinant 44 (CD44), intracellular adhesion molecule-1 (ICAM-1), and the receptor for hyaluronate-mediated motility (RHAMM) to effect anti-inflammatory and chondrogenic changes.

     
     
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     Platelet-rich plasma (PRP): peripheral blood undergoes centrifugation to produce concentrated sample of platelets. Endogenous (calcium chloride) or exogenous activators cause the release of plasma contents (growth factors, proteins, and chemokines). These factors may promote healing, reduce pain, and suppress inflammation. However, preparation techniques and compositions vary widely, which may lead to the inconsistent efficacy observed in the literature. ²

     
     
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     Bone marrow concentrates (BMCs): mesenchymal stem cells (MSCs) are isolated from BMCs. MSCs are chondrogenic, anti-inflammatory, and very effective as early as the first few weeks. Use of stem cell therapy for hip osteoarthritis and chondral defects is still in its infancy, offering promising short term results. ³

  
  
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  II. Microfracture: This is a single-step procedure used to treat chondral defect with full-thickness outer bridge grade 3 or 4 defect and no minimal evidence of osteoarthritis (Tonnis grade ≤ 1).4 Small puncture holes are created at the site of the chondral damage deep enough to allow for bone marrow bleed through the holes, which promotes healing. Fibrin adhesive is used to secure the chondral flap if applicable.

  
  
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  III. Autologous chondrocyte transplantation (ACT): this is a two-stage cartilage repair procedure. Viable chondrocytes are extracted from an individual, cultured, and then implanted. Indications include chondral defects ≥ 3 cm ² with focal, full-thickness outer bridge grade 3 or 4 defect and no minimal evidence of osteoarthritis (Tonnis grade ≤ 1).

  
  
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  IV. Autologous matrix-induced chondrocytes (AMIC) is a single-step procedure that recreates hyaline cartilage. Indications for AMIC are chondral defects ≥3cm ² with focal, full-thickness outer bridge grade 3 or 4 defect and no minimal evidence of osteoarthritis (Tonnis grade ≤ 1).

  
  
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  V. Osteochondral graft: the indication is subchondral plate involvement >0.5 cm ² thickness. The procedure includes surgical hip dislocation, femoral or acetabular lesion debridement completely exposing the subchondral bone, and finally the defect is packed with bone graft.

Implant Prosthesis

Hip implants for total hip arthroplasty consists of three parts: acetabular cup, femoral component, and the articular interface.

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  I. Acetabular cup: the part that fits into the acetabulum. Cups come as one-piece shells (monobloc) or modular.

  
  
  
  
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     One-piece shells: shells are either metal or ultra-high-molecular-weight polyethylene (UHMWPE). The metal cup is held in place by metal coating and UHM-WPE requires cement fixation.

     
     
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     Modular cups consist of a metal shell and liner. The outer part of the shell has porous coating for friction fitting. Two types of porous coating; foam metal and sintered beads, form friction fittings, which are designed to mimic the trabeculae of cancellous bone. Implant stability is determined by insertion force, under-rimming, and bone growth into the porous coating. ⁵

  
  
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  II. Femoral component: implant stem is fitted into the femur. The femoral head is attached to the stem. Stem fixation methods consist of cemented and uncemented fixations.

  
  
  
  
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     Cemented stems use acrylic bone cement to form mantle between the bone and stem. In uncemented stems, fixation is achieved by bone remodeling around the implant.

     
     
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     Femoral stems may be monolithic or modular. Modular components have variable head dimensions and neck orientations that permit variability in leg length, offset, and version.

  
  
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  III. Articular interface: it fits between the femoral head and the acetabular components. Interface size is determined by the outside diameter of the head or the inside diameter of the socket. ⁵