Adult Hip and Knee Reconstruction

Scott Ritterman, John Froehlich, and Matthew Miller

I. Total Hip Replacement

1. Anatomy

• Ball-and-socket joint

a. Socket deepened by thick flexible labrum

b. Cotyloid fossa: floor of acetabulum, no cartilage, ligamentum teres

◦ Radiographic marker “teardrop” is formed by:

♦ Quadrilateral plate from inside pelvis

♦ Cotyloid fossa from floor of acetabulum

c. Adult’s main blood supply to femoral head from medial femoral circumflex artery

• Surgical approaches

a. Anterior (Smith-Petersen approach)

◦ Internervous plane

♦ Sartorius (femoral nerve) and tensor fascia lata (superior gluteal nerve) superficially

♦ Gluteus medius (superior gluteal nerve) and rectus femoris (femoral nerve) deep

◦ Ascending braches of the lateral circumflex artery must be cauterized or ligated.

◦ Lateral femoral cutaneous nerve typically exits the pelvis laterally just inferior to the inguinal ligament, but there are several anatomic variants.

b. Anterolateral (Watson Jones)

◦ Intermuscular plane

♦ Tensor fascia lata and gluteus medius (superior gluteal nerve)

♦ Traditionally requires trochanteric osteotomy for adequate exposure of the hip joint

c. Direct lateral

◦ No intermuscular plane

♦ Splits anterior third of gluteus medius and minimus, and anterior portion of vastus lateralis

◦ Dislocate hip anteriorly

◦ Can lead to Trendelenburg gait postoperatively due weakness of gluteus medius and possible damage to superior gluteal nerve

♦ Superior gluteal nerve lies 3–5 cm proximal to greater trochanter on the underside of the gluteus medius muscle

♦ Prone to injury by stretching or transection

◦ No intramuscular plane

♦ Split gluteus maximus muscle (inferior gluteal nerve)

♦ Tag and divide piriformis tendon; alternatively, can spare tendon and retract

♦ Short external rotators are detached and reflected with posterior capsular flap used to protect the sciatic nerve during dissection

♦ Hip extension and knee flexion will protect the sciatic nerve from excess stretch intraoperatively.

◦ Increased dislocation risk if capsule is not repaired with the posterior approach

2. Physical examination

• Normal range of motion (ROM)

a. Flexion: 135 degrees

b. Extension: 10–15 degrees

c. Abduction: 45 degrees

d. Adduction: 30 degrees

e. Internal rotation: 35 degrees

f. External rotation: 45 degrees

• Motor function

a. Hip flexion against resistance from a seated position

◦ Test for iliopsoas impingement (snapping hip) with the patient seated, pain with resisted flexion

b. Adduction

c. Abductors: lay patient on contralateral side and keep leg abducted against resistance

d. Knee extension against resistance (quad strength)

• Gait examination

a. Antalgic: shortened stance phase, unloading painful limb

b. Abductor lurch (Trendelenburg gait)

◦ When the abductors are deficient, the pelvis tilts down toward the contralateral side during single-limb stance. In compensation, the trunk leans toward the deficient side to maintain the center of gravity over the weight-bearing limb. See Trendelenburg test, below.

• Leg length discrepancy (LLD)

a. Always should be evaluated preoperatively and intraoperatively

◦ Common cause of litigation

b. Some people have small LLD at baseline and most can tolerate up to 1 cm difference

c. True LLD

◦ Hip flexion/adduction contracture

◦ Scoliosis, pelvis obliquity

◦ Lengthening after long-term shortened limb (chronic femoral neck fracture, acetabular protrusio)

e. Measurements

◦ Anterior superior iliac spine (ASIS) to medial malleolus

◦ Umbilicus to medial malleolus

• Special tests

c. FABER (flexion, abduction, and external rotation) test: pain with flexion, abduction, external rotation in the SI joint region

3. Hip disease

• Dysplasia

a. Developmental malalignment of the hip joint

b. Typically presents in fourth to seventh decades with arthritis of the hip joint

c. May be subtle or pronounced

◦ Abnormal biomechanics lead to degenerative changes within the hip joint.

◦ Time of presentation depends on the degree of dysplasia.

d. Acetabular dysplasia

◦ Coverage

♦ Under-coverage of femoral head: typically anterior and laterally

♦ Ganz periacetabular osteotomy (PAO); used to correct acetabular dysplasia

▪ Osteotomize around the acetabulum while leaving the posterior column intact to redirect the native acetabulum.

▪ Goal is to medialize the joint and gain lateral/anterior coverage of the femoral head.

• Joint medialization decreases joint reactive forces.

• Correction of acetabular version, lateral center-edge angle

♦ Anterior hip decompression

▪ Arthroscopic versus open

▪ Debridement of cam lesion

▪ Debridement of anterior acetabulum (rim trim)

▪ Repair/debride torn labrum

♦ Proximal femoral osteotomy

▪ Varus producing osteotomy

• Typically done after PAO with insufficient lateral coverage (insufficient center-edge angle)

▪ Valgus producing osteotomy

• Typically for femoral neck nonunion in adults

♦ Total hip considerations with hip dysplasia

▪ Acetabulum is shallow, deficient superiorly and anteriorly.

▪ Femoral neck may be short with excessive anteversion.

▪ Proximal migration of femoral head may exist (chronic shortening of leg and neurovascular structures).

• Insufficient abductors may need to be transferred distally with trochanteric osteotomy.

• Shortened hamstrings and adductors may need to be lengthened.

• Shortened sciatic nerve

Susceptible to injury if leg lengthened > 2–3 cm, although can occur with less

May require femoral shortening osteotomy (subtrochanteric)

• Osteonecrosis

a. Loss of blood supply to femoral head

b. Posttraumatic

◦ Femoral head necrosis after femoral head or neck fracture, or traumatic or iatrogenic hip dislocation

c. Idiopathic: avascular necrosis (AVN)

◦ Demographics: third and fourth decades of life

◦ Presentation: acute or progressive groin pain, decreased/painful internal rotation and flexion

◦ Risk factors:

♦ Alcoholism

♦ Corticosteroids

♦ Hypercoagulability

▪ Sickle cell anemia

• 75% of asymptomatic patients with osteonecrosis develop symptoms/collapse if untreated

◦ Imaging

♦ Radiographs: AP pelvis, AP lateral hip

▪ Look for sclerosis or cystic changes in femoral head

▪ Bisphosphonates: can prevent/delay collapse if given in the early stages

♦ Surgical

▪ Core decompression

• Tunnel(s) drilled up the femoral neck into the head to allow decompression and revascularization; often combined with bone grafting

• May be effective early in disease before crescent sign and collapse occur (Ficat grades I and II)

▪ Vascularized fibular graft

• Free tissue transfer to place vascularized autograft into the femoral head

• Pain at donor site/leg following vascularized fibula transfer may indicate a tibial stress fracture; evaluate with MRI.

▪ Total hip replacement

• Most reliable treatment for AVN with collapse

• Young patient with Ficat grade III or IV (collapse present)

• Osteoarthritis

a. Progressive cartilage loss leads to weight bearing on subchondral bone

◦ Demographics: fourth to eighth decades of life; females > males

◦ Presentation: progressive groin pain with weight-bearing activities; decreased rotation; external rotation and flexion contractures due to long-standing joint inflammation

◦ Imaging

♦ Weight-bearing AP pelvis, AP hip, cross-table lateral of hip

◦ Treatment

♦ Nonsurgical

▪ Activity modification with continued low-impact exercise

▪ Weight loss

▪ NSAIDs

▪ Crutch/cane: placed in contralateral hand will off-load weight during stance phase

▪ Intra-articular corticosteroid injection (fluoro or ultrasound guided)

♦ Surgical

▪ Hip fusion

• Typical candidate is a young male laborer with debilitating arthritis who needs to continue working

• Unilateral disease

• One-third increase in energy needed to ambulate

• As with any fused joints, increased stresses are translated to adjacent joints, which become arthritic (spine, knee)

▪ Fusion to THA

• Often done for adjacent joint disease (lumbosacral spine most commonly, knee)

• Preoperative electromyogram (EMG) required to test abductors

• Deficient abductors require constrained hip liner or tripolar articulation

▪ Hip resurfacing arthroplasty

• Intended for active young male patients

• Larger head should confer greater static stability

• Metal-on-metal resurfacing of femoral head and acetabulum

• Advantages include preservation of bone stock, lower wear without polyethylene liner, retain large femoral head size and therefore lower dislocation risk

• Contraindications

Osteoporosis, coxa vara, femoral neck cyst

Abnormal acetabular anatomy

Significant leg length discrepancy

Smaller anatomy

Renal disease

Metal allergy

• Complications

Periprosthetic femoral neck fracture up to 4% and is the most common reason for revision before 20 weeks

Metal-on-metal debris is generated, leading to T-cell immune response; see Metal-on-metal wear, below.

• Hemiarthroplasty

a. Typically used for femoral neck fractures in low demand, elderly patients

c. Low demand, infirm patient without antecedent hip/groin pain

d. Advantage is large head size, allowing for more stability and lower dislocation rate

◦ Preserve the labrum, repair hip capsule for greatest postoperative stability

e. Active patients develop groin pain due to chondrolysis and have superior outcome with total hip replacement

• Total hip arthroplasty

a. Use THA for active patients with arthritis and displaced femoral neck fracture.

b. Higher dislocation rate with femoral neck fracture than elective THA for osteoarthritis (OA)

4. Basics of total hip replacement

• Bearing surfaces

a. Soft: ceramic on polyethylene (PE), metal on PE

b. PE: hydrocarbon molecule

◦ Ultra-high molecular weight polyethylene (UHMWPE) has been in use for > 40 years

♦ Mechanical properties depend on percentage of PE in

▪ Amorphous phase

▪ Crystalline phase

▪ Better wear characteristics

♦ Significantly reduces wear compared with conventional UHMWPE

▪ Less mechanical strength

♦ Need to find compromise between the two

◦ Manufacturing

♦ Direct compression molding from powder has the best wear characteristics.

◦ Sterilization

♦ Low-dose irradiation (2.5–4 Mrd) is best mode

♦ Irradiation (at higher dose) also allows cross-linking to occur

◦ Cross-linking of hydrocarbon chains between molecules of PE provides greater wear resistance.

◦ Irradiation of PE creates free radicals that can bind either:

♦ Oxygen: oxidized PE, chain scission, and no cross-linking occurs; greatly increased wear

♦ Other PE molecules: in an oxygen-free environment (typically in the presence of inert gases such as argon), free radicals bind other PE molecules and form cross-links (which is good)

◦ Therefore, irradiation must be done in an oxygen-free environment!

◦ Irradiation in larger amounts of radiation (5–20 Mrad) creates highly cross-linked polyethylene

♦ Better wear resistance than cross-linking with lower dose of irradiation

♦ Decreased mechanical strength compared with regular cross-linked PE, more brittle

♦ Smaller wear particles

◦ Heating of PE after cross-linking is required to remove excess free radicals. Melting removes all free radicals; annealing removes some.

♦ Affects the structure of PE

▪ Crystalline: ideally 45–65% of PE

• Greater percentage of crystallinity leads to greater mechanical strength

▪ Amorphous

• Where cross-linking occurs

♦ Melting

▪ Less free radicals, better resistance to oxidation in vivo

▪ Reduces mechanical properties; lowers crystallinity

♦ Annealing: heating PE to less than melting point

▪ Better mechanical properties than melting (decreased wear) due to higher crystallinity

◦ Vitamin E can be added to reduce free radicals by blending or infusion.

◦ Shelf life: vacuum-sealed package

♦ Remaining free radicals have the potential to oxidize while PE sits on the shelf awaiting use.

♦ Concern about odd-sized components that may sit on shelf for extended time

◦ All PE components regardless of means of sterilization will begin to oxidize once implanted and exposed to environment.

◦ PE wear products lead to osteolysis (see below)

c. Hard bearing surfaces

◦ Metal

♦ Alloys consisting of cobalt, chrome, molybdenum, nickel, and other substances are used for bearing surfaces due to their resistance to corrosion.

♦ Titanium is too soft to use as a bearing surface but has a stiffness similar to bone and is therefore ideal for use in both femoral implant and acetabular shell.

▪ Titanium has a Young modulus of 115 GPa.

♦ Scratches too easily, leading to increased wear if used for bearing surface (head)

◦ Theoretically, metal-on-metal (MOM) bearings have a lower wear rate, generate smaller wear particles compared with PE bearings, and allow the use of a large head, conferring greater joint stability.

◦ Imprecise placement of components, especially an over-abducted or anteverted cup, can lead to edge loading and generation of a large number of wear particles, increasing serum cobalt and chromium ion concentrations, and stimulating a T-cell response (see Metal-on-metal wear, below).

◦ Ceramic: alumina ceramic and zirconia ceramic

♦ Ceramic on ceramic (COC)

▪ Low wear

▪ Fewer particles than in MOM

▪ Bio-inert debris

▪ Limited head size, less optimal fluid film mechanics (see below)

▪ Can entail squeaking, possibly due to component malposition

• First-generation ceramic implants susceptible to fracture, due to manufacturing imperfections and brittle materials

• Current-generation ceramic implants have significantly lower fracture rates

f. Radial clearance

◦ Difference in radius of curvature of the head and the cup

♦ Equatorial contact

▪ Head is larger than the cup.

▪ High friction

• No space for fluid to enter or exit

♦ Polar contact

▪ Head is smaller than the cup.

▪ One point of contact

▪ High stress at point of contact and poor lubrication

♦ Midpolar contact (ideal)

▪ Radius of curvature of head is slightly smaller than that of cup.

▪ Fluid is able to enter the joint and lubricate the bearing surfaces.

▪ Cannot have complete congruence or fluid cannot enter to lubricate joint

g. Wear

◦ Any process that leads to breakdown of bearing surfaces, resulting in particulate debris, increased friction, and altered biomechanics

◦ Volumetric wear

♦ Calculated, though exact equation is debated

♦ Directly related to size of prosthetic head

♦ Larger heads lead to greater volumetric wear

◦ Linear wear

♦ Wear rates above 0.1 mm per year are at high risk of osteolysis.

▪ Any new polyethylene with wear rates < 0.1 mm/year should have minimal osteolysis (> 10 billion particles/gram of tissue)

♦ Measured on radiograph

♦ Penetration of femoral head into the liner

◦ Adhesive wear

♦ PE particles pulled off form liner during gait cycle

♦ Most important in generation of PE debris in hips

◦ Abrasive wear

♦ Rough femoral head scratches and mechanically damages PE liner

◦ Third body wear

♦ Any material between the two bearing surfaces; cement, metal shavings, bone, etc.

♦ Debris between two bearing surfaces wears/scratches the weaker one

◦ Run-in wear

♦ Higher wear rate within the first 1 million cycles of THA use (bedding in period)

♦ Decreases thereafter as it goes into steady-state phase

♦ High stress points on surfaces polished out

◦ Stripe wear

♦ Seen with ceramic prosthetic heads

♦ Occurs with lift-off separation when the femoral head contacts the rim of the shell as it separates from the socket

♦ Surface wear 1–60 µm deep

♦ Seen more commonly in those cups that are vertically oriented

◦ Hip edge loading: radiograph shows an over-abducted cup

♦ Stresses concentrate on edge of acetabular shell

• Implant fixation

a. Biological

◦ Biological fixation: dynamic relationship between bone and prosthesis with ability to remodel over time

◦ Prosthesis must be coated to allow bone ingrowth or ongrowth.

◦ Beware of devascularized bone.

♦ Patients who have been irradiated may not be able to support bony ingrowth (requiring tantalum cup and multiple screw fixation or cement for fixation).

◦ Press fit versus line-to-line technique

♦ Press fit refers to preparing bone (either femur or acetabulum) to a certain size and then inserting an implant that is slightly larger (1–2 mm). Compressive hoop stresses provide the initial fixation while bone ongrowth/ingrowth occurs.

♦ Line-to-line technique refers to preparing the femur or cup to a certain size and inserting the same-size implant. Acetabular shell requires screws if placed in this fashion; femoral stem is fully porous coated for initial rigid fixation.

▪ For both, the long-term fixation is biological.

◦ Porous coating: allows for bone ingrowth

♦ Porosity

▪ Too little porosity will not leave enough room for ingrowth.

▪ Too much porosity will lead to failure due to shear forces.

▪ 40–50% porosity is ideal

• Pore depth

Deeper is better to some extent.

• Pore size

50–400 µm is ideal pore size

• Micromotion

< 30 µm for bone ingrowth

> 150 µm leads to fibrous fixation

• Proximal or metaphyseal loading

Stems that are coated proximally will lead to proximal ingrowth and loading of proximal bone (preventing stress shielding).

◦ Grit blasting: allows for bone ongrowth

♦ Surface roughness

▪ Difference in height between peaks a valleys

▪ Rougher the surface, greater the fixation

♦ Typically diaphyseal fitting with more stress shielding proximally because of diaphyseal loading/stress

▪ Fully coated stems will gain fixation distally into the diaphysis.

▪ Stress shielding may occur, which leads to decreased bone density and remodeling proximally.

♦ More function of stem stiffness

▪ Thicker stems

▪ Cobalt is stiffer than Titanium, and Titanium is stiffer than cortical bone

♦ Hydroxylapatite-coated stems have shorter time frame to achieve biological fixation.

▪ Ca₁₀(PO₄)₆(OH)₂

b. Cemented

◦ Static fixation, diaphyseal loading

◦ Relies on interdigitation between cement and trabecular bone

◦ Long-term failure in young active patients

♦ Will fatigue at stress points in mantle; does not remodel

♦ Cemented cups fail earlier than femur

▪ Too much shear and tensile forces

▪ Cement stronger in compression, fails in tension/shear

◦ The femur must be appropriately broached and cleared of any marrow or fat debris.

◦ A canal restrictor is placed within the femoral canal 1.5–2 cm distal to the eventual tip of the prosthesis.

◦ The canal is back-filled with cement from distal to proximal.

♦ The cement is pressurized.

♦ Component is centralized in the cement mantle and held in position until cement hardening.

♦ Cement mantle should be at least 2 mm on every side and cement should occupy two thirds of the canal and the prosthesis the remaining one third.

♦ A mantle defect is a point where the prosthesis touches the bone and is a high stress point, susceptible to fracture.

♦ Osteopenic bone has improved fixation with cemented components. Porous bone allows increased interdigitation of cement.

• Acetabular component screw placement

a. Required for line-to-line acetabular component placement

b. Line between ASIS and center of fovea splits acetabulum into halves

c. Line two is perpendicular at the center of the acetabulum, dividing into four quadrants