and Michael Cross5
(5)
Department of Orthopaedic Surgery, Hospital for Special Surgery, 535 E 70th Street, 10021 New York, USA
(6)
Department of Orthopaedic Surgery, Rush University Hospital, 1611 W Harrison Street, Chicago, IL 60612, USA
Take-Home Message
Preoperative planning is essential to obtain consistent, reproducible outcomes and to restore leg lengths and offset.
Generally, the goal of acetabular component position should be 35–45° of abduction and 10–25° anteversion.
The goal of femoral component position should be 10–15° of anteversion.
Cementless hips (particular acetabular components) have decreased rates of aseptic loosening compared to cemented components in younger patients.
Cemented or hybrid fixation carries the risk of fat embolism syndrome, intraoperative hypoxia, and hypotension.
Definitions
Primary arc range – arc of motion prior to impingement and subsequent dislocation.
Excursion – distance a head must displace to dislocate.
Press–fit (underream) – prepare the cup or canal 1–2 mm smaller than the implant diameter to allow the bone to expand.
Line to line – bone is prepared to actual size of implant.
Radiography
Preoperative: AP Pelvis, AP/lateral hip (cross-table or frog lateral).
AP views should be taken with hip/foot in 10°–15° of internal rotation to get true AP view of femoral neck offset (assuming normal anteversion of 10°–15°).
Templating
Order is the same as surgery – acetabular side first, then femoral component second
Acetabular side:
Draw horizontal reference line through base of both teardrops.
Mark the ilioischial line, teardrop, and superolateral margin of acetabulum.
Place cup template at 40°+/–10° of abduction with the medial border at the ilioischial line, ensuring there is adequate lateral bone coverage.
If templating for a cemented cup, allow for 2–3 mm cement mantle.
Mark center of rotation of cup and compare to contralateral side.
Special consideration is necessary for protrusio acetabuli or a lateralized or dysplastic acetabulum.
Femoral side:
Goal – restore femoral offset and optimize limb length.
Determine limb length discrepancy by measuring distance from lesser trochanter (or tip of greater trochanter) to horizontal reference line – compare this to discrepancy seen on physical exam (measure from ASIS to medial malleolus to determine limb length or use block testing which measures functional leg length discrepancy).
During templating, if the center of rotation of the femur is superior to that of the acetabulum, you will increase limb length; conversely if it is inferior, you will shorten the limb.
When templating width of stem, for cementless proximally fitted stem, you want optimal contact between the lateral and medial endosteal cortices of the proximal femur.
For fully porous-coated stems, it’s necessary to obtain optimal endosteal contact in the diaphysis.
In cemented stems, allow for a 2 mm circumferential cement mantle.
Restoring offset – goal is to restore offset of normal hip.
If templated center of rotation of the prosthetic head is medial to the cup, this will produce an increased offset. Conversely, if it’s lateral to the cup, there will be decreased offset.
Hip Stability
Component design and alignment
Assess stability intraoperatively by taking hip through extreme ROM.
Design:
Primary arc range – arc of motion prior to impingement and dislocation; determined by head-neck ratio (larger ratio = greater arc ROM before impingement) – narrow neck tapers (e.g., 12/14) have a more favorable head–neck ratio and therefore are more stable than a neck with a collar/skirt.
Excursion – distance a head must displace to dislocate (smaller the excursion, the easier a hip will dislocate); this distance is one half the femoral head diameter (28 mm head has an excursion of 14 mm).
Alignment – THA femoral head is smaller than native, causing decreased stability and ROM.
You want the THA primary arc ROM within the center of the patient’s functional ROM. This way there will be some stability if the primary arc ROM is exceeded.
If the THA is malaligned, the primary arc is not centered, leading to excessive hip excursion on one side of the arc of ROM.
Types of Fixation
Cup position should be: abduction of 35°–45° and anteversion of 10°–25°.
Femoral stem should have 10°–15° anteversion.
Stem can often be placed in the patient’s anatomic version, unless the patient has excessive anteversion or femoral retroversion.
Cemented – essentially a static fixation without potential to remodel.
Mechanical stability of cemented implants increases with worsening osteoporosis as the cement is better able to interdigitate with more porous bone.
Historically a cement mantle of 2 mm circumferentially around implant was recommended; more recently the two thirds rule is often applied.
Two thirds of the canal is filled with the implant, and one third is filled with cement.
Cement mantle defects (bone touches prosthesis) create an area of concentrated stress, leading to earlier loosening.
Stiff implants are preferred to avoid bending/torsional forces on cement mantle.
Cementless (biologic) – dynamic biologic fixation with ability to remodel.
Arguably, should not be used in bone that was previously irradiated as this bone will have decreased bone ingrowth/ongrowth, leading to aseptic failure.
Initially ingrowth requires the implant to be in contact with cortical (not cancellous) bone, surface coating/ongrowth surface +/− with hydroxyapatite (osteoconductive; rapidly closes gaps), and rigid fixation (micromotion less than 150 um) to allow bone in/ongrowth.
Rigid fixation is achieved by one of two techniques:
Press-fit (underream) – prepare the cup 1–2 mm smaller than the implant diameter to allow the bone to expand (generates hoop stresses around implant to prevent motion) when the implant is inserted – common for acetabular preparation depending on the system used.
Line to line – bone is prepared to actual size of implant.
Porous coating
Metallic surface has pores (sized between 50–350 um) into which bone can grow.
If pore size is too big/small, bone ingrowth will be suboptimal.
Deeper pores allow increased bone ingrowth.
40–50 % of surface should be porous to allow bone to fill a significant amount of the stem.
Less porosity inhibits bone ingrowth, and excessive porosity increases risk of the porous-coated surface shearing off.
Grit blasted
Abrasive spray roughens up metal surface, producing hills and valleys on surface into which bone can grow and produce a stable construct
Bone ongrowth increases with increasing surface roughness (distance between peak and valley on surface).
Unlike the porous-coated implant where bone grows into the prosthesis, bone only grows onto the prosthesis with grit blasted (or plasma spray) implants.
This necessitates a larger area of coating to obtain stability.
Complications
Loosening
Aseptic femoral/acetabular loosening is a primary reason for revision THA.
In cemented THA, the acetabular component often fails.
In cementless THA, the femoral component more often fails.
Evaluate with radiographs – presence of following predicts loosening.
Presence of radiolucent lines around the components
Prosthesis fracture
Presence of a pedestal
Subsidence or change in the position of the components over time when looking at successive radiographs
Lack of spot welds
Lack of medial calcar blunting
Dislocation – cited at 1–2 % (some cite up to 9 %) after primary and up to 25 % after revision
Leg-length discrepancy
True discrepancy – transverse ischial line on AP pelvis radiograph shows a discrepancy.
Apparent discrepancy – leg length is restored and is equal as measured by a the transverse ischial line; however, because of abductor contracture or scoliosis, the operative leg feels different.
This will resolve unless it is secondary to severe scoliosis.
Do not use a shoe lift for at least 3–6 months after surgery.
Most common reason for litigation after total hip replacement.
Stress shielding – bone density loss in proximal femur over time in a patient with a well-fixed implant.
Caused by stiff implants (stiffness increases with the stem radius (r 4) and with round, solid stems) bearing majority of stress/weight.
Hollow/tapered and fluted stems are less stiff.
Usually with cementless femoral components.
Most common with fully porous-coated stems.
Fat embolism syndrome – rapid onset of hypotension and hypoxia shortly after cemented implant is inserted.
Most commonly seen when inserting the cemented stem (i.e., during femoral preparation) as opposed to the cup.
Fat is forced out of bone and into the venous system by the increase in intramedullary pressure.
Fat droplets reach lungs and act as small emboli, preventing oxygenation of blood returning to the lungs.
Vasodilatation, leading to hypotension, also occurs.
Seen more commonly in elderly patients (usually because they have cardiopulmonary disease and have osteoporotic, porous bone which allows fat to escape more easily).
Bibliography
1.
Barrack RL. Dislocation after total hip arthroplasty: implant design and orientation. J Am Acad Orthop Surg. 2003;11(2):89–99. Epub 2003/04/03.
2.
Corten K, Bourne RB, Charron KD, Au K, Rorabeck CH. Comparison of total hip arthroplasty performed with and without cement: a randomized trial. A concise follow-up, at twenty years, of previous reports. J Bone Joint Surg Am. 2011;93(14):1335–8. Epub 2011/07/28.
3.
Della Valle AG, Padgett DE, Salvati EA. Preoperative planning for primary total hip arthroplasty. J Am Acad Orthop Surg. 2005;13(7):455–62. Epub 2005/11/08.
4.
Emerson Jr RH. Increased anteversion of press-fit femoral stems compared with anatomic femur. Clin Orthop Relat Res. 2012;470(2):477–81. Epub 2011/07/26.
5.
Kiernan S, Hermann KL, Wagner P, Ryd L, Flivik G. The importance of adequate stem anteversion for rotational stability in cemented total hip replacement: a radiostereometric study with ten-year follow-up. Bone Joint J. 2013;95-B(1):23–30. Epub 2013/01/12.
6.
Ritter MA, Harty LD. Fat embolism in revision total hip arthroplasty. J Arthroplasty. 2002;17(8):1063–5. Epub 2002/12/13.
7.
Takenaga RK, Callaghan JJ, Bedard NA, Liu SS, Klaassen AL, Pedersen DR. Cementless total hip arthroplasty in patients fifty years of age or younger: a minimum ten-year follow-up. J Bone Joint Surg Am. 2012;94(23):2153–9. Epub 2012/12/12.
2 Tribology
Alexander Christ7
(7)
Department of Orthopaedic Surgery, Hospital for Special Surgery, 535 E 70th Street, 10021 New York, USA
Take-Home Message
Tribology is the study of friction, wear, and lubrication.
The most common type of wear in total joint arthroplasty is adhesive wear.
Three-body wear is a type of abrasive wear.
Although 200 times smoother, prosthetic joints do not have as low a friction coefficient as native joints due to the complex interactions between articular cartilage and synovial fluid, which contains hyaluronate molecules that allow it to act as a non-Newtonian fluid.
Definitions
Tribology: the study of friction, wear, lubrication, and their interrelationships. Derived from Greek term “tribos,” meaning rubbing
Bio–tribology: the study of tribology of biologic systems, in this case synovial joints, with the goal of understanding these systems and developing medical interventions to address pathology
History
Early 1500s, da Vinci first relates friction as ratio of transverse force to axial load.
1699 Amontons describes friction as independent of contact area.
1781 Coulomb describes friction independent of velocity of surfaces.
1962 Charnley performs total hip arthroplasty with stainless steel femoral head and ultra-high-molecular-weight polyethylene cup.
1966 Jost publishes landmark report first coining term “tribology.”
1973 Dowson opens field of bio-tribology with the study of lubricants in nature.
Basic Principles
Friction – defined as the resistance when two bodies slide across each other. The coefficient of friction, μ, relates the transverse resistive load to axial loading. All surfaces exhibit some roughness (small variations in surface height), and the projections of a surface are referred to as asperities. Friction is formed as asperities slide past each other and deform.
Wear – the fundamental microscopic process by which material is removed from a surface. This can be mechanical or chemical. Wear mechanisms are often classified as follows:
Abrasive: hard particles/protuberances are forced to move against and along each other. Three–body wear is the most common form of abrasive wear in arthroplasty, where loose bodies between the two surfaces cause accelerated polyethylene wear.
Adhesive: generated by the sliding of one surface along another, where asperities fuse and are subsequently ruptured. This is the most common form of wear in arthroplasty.
Fatigue: occurs when the surface of a material is deformed by cyclic loading. This type of wear is asymmetric.
Fretting: caused by small amplitude oscillatory movement between contact surfaces.
Erosive: caused by particles that impinge on a surface or edge and remove material.
Corrosive: due to chemical reaction on a wearing surface. Most common form is oxidation, where metal oxides have much different shear strengths and tend to flake away.
Lubrication – a material included between surfaces that reduces friction. Three basic modes:
Fluid Film: two surfaces separated by a thin fluid layer so that asperities do not contact.
Boundary layer: monolayer of lubricant molecules absorb to each surface. This occurs at low velocities and high loading.
Mixed: between the extremes of fluid film and boundary.
Running–in – occurs when newly machined surfaces experience a decrease in friction coefficient as they go from mixed to fluid film lubrication. This has been demonstrated in metal-on-metal prostheses.
Characteristics of Joints
Native Joint
Articular cartilage – 65–80 % water, 10–20 % collagen (type II), and 4–7 % aggrecan
Surface roughness of 1–6.0 μm, coefficient of friction 0.005–0.02 for knees, and 0.01–0.04 for hips
Lubricated by synovial fluid, which is non-Newtonian (nonconstant viscosity) due to long-chain hyaluronate molecules, making it a much more effective lubricant.
Rheumatoid synovial fluid loses hyaluronate molecules (less effective lubricant).
Osteoarthritic synovial fluids maintain hyaluronate component (still an effective lubricant).
Continual loading causes fatigue damage and interfacial wear.
Prosthetic Joint
Much smoother than native joint (~0.0025 μm) but similar coefficient of friction 0.015–0.03.
Lubrication provided by third-space fluid and lymphatic sources.
Also experience fatigue damage and interfacial wear.
Metal-on-metal joints undergo a running-in period, while ceramic on ceramic does not.
Bearing Surfaces
Materials
Polymers – the most commonly used is cross-linked ultra-high-molecular-weight polyethylene.
Ceramics – aluminum oxide and zirconia-toughened aluminum.
Metals – cobalt-chrome-molybdenum alloys.
Combinations
Hard on soft: metal or ceramic head with poly cup
Hard on hard: ceramic on ceramic or meta on metal
Simulation
Wear Screening Device
Provides information exclusively on intrinsic features of materials.
Do not represent geometry of biomaterials.
Inadequate at predicting in vivo wear rates.
Much faster and cheaper.
Wear Joint Device
Real prostheses tested in an environment simulating physiological conditions.
Multiple parameters: degrees of freedom (DOF), stations, loading, ball-cup position, lubrication fluid, and temperature.
Two–axis simulator reproduces flexion/extension and abduction/adduction.
Three–axis simulator adds internal/external rotation but is more expensive and computationally more complicated.
Bibliography
1.
Affatato S, Spinelli M, Zavalloni M, Mazzega-Fabbro C, Viceconti M. Tribology and total hip joint replacement: current concepts in mechanical simulation. Med Eng Phys. 2008;30(10):1305–17.
2.
Bhushan B. Definition and history of tribology. In: Principles and applications of tribology. New York: Wiley; 1999.
3.
Davin JP. Biotribology. Hoboken: Wiley; 2010.
4.
Dowson D. Bio-tribology. Faraday Discuss. 2012;156:9–30; discussion 87–103.
5.
Mazzucco D, Spector M. The role of joint fluid in the tribology of total joint arthroplasty. Clin Orthop Relat Res. 2004;429:17–32.
6.
Norris JA, Stabile KJ, Jinnah RH. An introduction to tribology. J Surg Orthop Adv. 2008;17(1):2–5.
3 Osteolysis and Wear
Michael B. Cross8 and Wayne G. Paprosky9
(8)
Department of Orthopaedic Surgery, Hospital for Special Surgery, 535 E 70th Street, 10021 New York, USA
(9)
Department of Orthopaedic Surgery, Rush University Hospital, 1611 W Harrison Street, Chicago, IL 60612, USA
Take-Home Message
Osteolysis occurs from a cellular response to wear debris and particles.
Osteolysis is best diagnosed using serial x-rays after total hip arthroplasty.
C-reactive protein (CRP) and erythrocyte sedimentation rate (ESR) should be performed to rule out a septic joint prior to any revision for osteolysis.
The 4 main indications to revise a patient for polyethylene wear are: severe pain, instability, full-thickness polyethylene wear, and rapid and progressive osteolysis seen on serial x-rays.
Definitions
Osteolysis – bone loss secondary to a cellular response to wear debris and particles
Etiology
Cellular response to wear debris and particles, most commonly from polyethylene (macrophages) but can also occur from metal particles (lymphocytes)
Pathophysiology
Progressive wear of the polyethylene liner generating ~1 μm particles.
Macrophages engulf the particles and activate osteoclasts.
Osteoclastic response to wear particles (most commonly polyethylene).
Bone resorption.
Radiology
Prior x-rays, including the initial postoperative x-ray are helpful.
AP pelvis, cross-table lateral (or frog lateral), and Judet views.
CT scan or MRI is also helpful to determine extent of osteolysis.
Classification
Type I: stable and well-positioned modular cup with an intact locking mechanism
Type II: any of the following, unstable (loose) cup, poorly positioned acetabular component, nonmodular cup, and poor track record of the cup
Indications
With any revision THA, deep infection should be ruled out preoperatively by obtaining an erythrocyte sedimentation rate (ESR) and C-reactive protein (CRP).
If ESR or CRP is elevated, a hip aspiration should be performed to obtain a cell count with differential and aerobic/anaerobic cultures.
The indications to manage polyethylene wear and osteolysis:
Severe pain, instability, full-thickness polyethylene wear with metal head articulating with the metal socket, and/or rapid and progressive osteolysis seen on serial x-rays
Treatment
Yearly monitoring of radiographs until indications for surgery are met
Once surgery indications are met:
Type I: polyethylene liner exchange with larger head if possible
Type II: requires acetabular component revision
Prior to surgery, one must find out the implant manufacturer, size of the shell, femoral stem type, and femoral head size allowed with acetabular component size.
Complications
Main complication is dislocation after head and liner exchange.
Bibliography
1.
Maloney WJ, Herzwurm P, Paprosky W, Rubash HE, Engh CA. Treatment of pelvic osteolysis associated with a stable acetabular component inserted without cement as part of a total hip replacement. J Bone Joint Surg Am. 1997;79(11):1628–34.
2.
Maloney WJ, Paprosky W, Engh CA, Rubash H. Surgical treatment of pelvic osteolysis. Clin Orthop Relat Res. 2001;393:78–84.
3.
McArthur B, Cross M, Geatrakas C, Mayman D, Ghelman B. Measuring acetabular component version after THA: CT or plain radiographs? Clin Orthop Relat Res. 2012;470(10):2810–8.
4.
Puri L, Lapinski B, Wixson RL, Lynch J, Hendrix R, Stulberg SD. Computed tomographic follow-up evaluation of operative intervention for periacetabular lysis. J Arthroplasty. 2006;21(6 Suppl 2):78–82.
5.
Restrepo C, Ghanem E, Houssock C, Austin M, Parvizi J, Hozack WJ. Isolated polyethylene exchange versus acetabular revision for polyethylene wear. Clin Orthop Relat Res. 2009;467(1):194–8.
6.
Schmalzried TP, Fowble VA, Amstutz HC. The fate of pelvic osteolysis after reoperation. No recurrence with lesional treatment. Clin Orthop Relat Res. 1998;350:128–37.
7.
Southwell DG, Bechtold JE, Lew WD, Schmidt AH. Improving the detection of acetabular osteolysis using oblique radiographs. J Bone Joint Surg Br. 1999;81:289–95.
8.
Stulberg SD, Wixson RL, Adams AD, Hendrix RW, Bernfield JB. Monitoring pelvic osteolysis following total hip replacement surgery. An algorithm for surveillance. J Bone Joint Surg Am. 2002;84-A(Suppl 2):116–22.
9.
Talmo CT, Kwon YM, Freiberg AA, Rubash HE, Malchau H. Management of polyethylene wear associated with a well fixed modular cementless shell during revision total hip arthroplasty. J Arthroplasty. 2011;26(4):576–81.
4 Polyethylene Manufacturing
Benjamin F. Ricciardi10
(10)
Department of Orthopaedic Surgery, Hospital for Special Surgery, 535 E 70th Street, 10021 New York, USA
Take-Home Message
Ultra-high-molecular-weight Polyethylene is a long–chain polymer of ethylene.
Primary fabrication includes direct compression molding or ram extrusion.
Sterilization methods include radiation, ethylene oxide, and gas plasma.
Radiation results in cross–linking through free radical production. These free radicals can be quenched with annealing or remelting.
Highly cross–linked polyethylene (UHMWPE) has decreased wear but worse mechanical properties (lower fracture toughness) than conventional polyethylene in vitro and in total hip arthroplasty at medium-term follow-up.
Definitions
Polyethylene: a long-chain polymer of the monomer hydrocarbon ethylene
Ultra-high-molecular-weight polyethylene (UHMWPE): long polyethylene polymer with molecular weight of between 1 and 6 million g/mol in molecular weight
Fabrication Methods
UHMWPE is primary polyethylene traditionally used with orthopedic devices.
Three primary methods of UHMWPE fabrication:
Direct molding – resin is placed in mold that compresses to create component. Also called net shape molding.
Ram extrusion – resin is first extruded under heat and pressure into a cylindrical bar, which is then machined into final component.
Compression molding – resin first molded into large sheet, which is then machined to create final component.
Direct compression molding: combines compression molding with direct molding to create polyethylene component directly from a mold under high heat and pressure.
Sterilization and Degradation
Until 1995, UHMWPE was sterilized for clinical use with low-dose (25– 40 kGy) gamma radiation in air and packaged in air.
Gamma irradiation in air created free radicals that are prone to oxidize in the polyethylene with treatment and shelf aging, causing early and rapid degradation of physical, chemical, and mechanical properties, leading to catastrophic failure. As a result, alternative sterilization methods were created.
Alternative sterilization methods that reduce oxidation: gamma radiation with inert or low oxygen package ethylene oxide gas plasma
Cross-Linking
Radiation can create cross–linking within the polyethylene through recombination of free radicals (not seen in ethylene oxide or gas plasma sterilization alone).
Advantage: cross–linking reduces abrasive and adhesive wear of polyethylene.
Disadvantage: may decrease tensile strength and resistance to crack propagation.
Type of radiation (gamma, electron beam), dose (50–100 kGy), atmosphere (air, argon, nitrogen), and postradiation treatment (annealing, remelting, sequential radiation) can all influence amount of cross-linking and effect on mechanical properties.
Postradiation processing: helps to reduce residual free radicals left after radiation, preventing degradative effects.
Remelting: heating above melting temperature (>140 °C). Eliminates all free radicals but decreases mechanical properties through reduced crystallinity.
Annealing: heating to just below melting temperature. Leaves more free radicals than remelting but has less effect on mechanical properties, and thus, it is thought a thinner polyethylene can be tolerated.
Vitamin E: antioxidant that may protect against oxidation and avoid need to melt polyethylene.
First-generation highly cross-linked HMWPE: postirradiation remelting or annealing performed to quench free radicals.
Second-generation highly cross-linked HMWPE: tried to improve remelting effect on mechanical properties or annealing with residual free radicals. All avoid melting. Includes sequential annealing and irradiation, vitamin E, and mechanical deformation with annealing.
Clinical Results: THA
In vitro: mechanical testing revealed decreased wear of highly cross–linked polyethylene versus conventional polyethylene. Additionally, highly cross-linked polyethylene linear wear rates appear independent of head size, unlike conventional polyethylene, where increased head size creates more volumetric wear.
In vivo: Lower femoral head penetration rates, significantly reduced volumetric wear, and reduced rates of radiographic osteolysis with highly cross-linked polyethylene for acetabular components up to 13 years follow-up.
Clinical Results: TKA
In vitro: mechanical testing revealed decreased wear of highly cross-linked polyethylene versus conventional polyethylene. Smaller, more biologically active particles produced by wear of highly cross–linked polyethylene in knee simulator relative to conventional polyethylene.
In vivo: Less clinical evidence for reduced wear or osteolysis with highly cross-linked polyethylene in TKA. Post fracture may be a concern given reduced mechanical properties in posterior-stabilized knee designs. Need more clinical evidence to determine the benefits.
Bibliography
1.
Crowninshield RD, Muratoglu OK; Implant Wear Symposium 2007 Engineering Work Group. How have new sterilization techniques and new forms of polyethylene influenced wear in total joint replacement? J Am Acad Orthop Surg. 2008;16(Suppl 1):S80–5.
2.
Digas G, Kärrholm J, Thanner J, Malchau H, Herberts P. The Otto Aufranc Award. Highly cross-linked polyethylene in total hip arthroplasty: randomized evaluation of penetration rate in cemented and uncemented sockets using radiostereometric analysis. Clin Orthop Relat Res. 2004;429:6–16.
3.
D’Lima DD, Hermida JC, Chen PC, Colwell Jr CW. Polyethylene cross-linking by two different methods reduces acetabular liner wear in a hip joint wear simulator. J Orthop Res. 2003;21(5):761–6.
4.
Engh Jr CA, Hopper Jr RH, Huynh C, Ho H, Sritulanondha S, Engh Sr CA. A prospective, randomized study of cross-linked and non-cross-linked polyethylene for total hip arthroplasty at 10-year follow-up. J Arthroplasty. 2012;27(8 Suppl):2–7.e1.
5.
Engh CA, Sychterz CJ, Engh Jr CA. Conventional ultra-high molecular weight polyethylene: a gold standard of sorts. Instr Course Lect. 2005;54:183–7.
6.
Hopper Jr RH, Young AM, Orishimo KF, Engh Jr CA. Effect of terminal sterilization with gas plasma or gamma radiation on wear of polyethylene liners. J Bone Joint Surg Am. 2003;85-A(3):464–8.
7.
Kurtz SM, Muratoglu OK, Evans M, Edidin AA. Advances in the processing, sterilization, and crosslinking of ultra-high molecular weight polyethylene for total joint arthroplasty. Biomaterials. 1999;20(18):1659–88.
8.
Lachiewicz PF, Geyer MR. The use of highly cross-linked polyethylene in total knee arthroplasty. J Am Acad Orthop Surg. 2011;19(3):143–51.
9.
Li S, Burstein AH. Ultra-high molecular weight polyethylene. The material and its use in total joint implants. J Bone Joint Surg Am. 1994;76(7):1080–90.
10.
McCalden RW, MacDonald SJ, Rorabeck CH, Bourne RB, Chess DG, Charron KD. Wear rate of highly cross-linked polyethylene in total hip arthroplasty. A randomized controlled trial. J Bone Joint Surg Am. 2009;91(4):773–82.
11.
Micheli BR, Wannomae KK, Lozynsky AJ, Christensen SD, Muratoglu OK. Knee simulator wear of vitamin E stabilized irradiated ultrahigh molecular weight polyethylene. J Arthroplasty. 2012;27(1):95–104.
12.
Muratoglu OK, Merrill EW, Bragdon CR, O’Connor D, Hoeffel D, Burroughs B, Jasty M, Harris WH. Effect of radiation, heat, and aging on in vitro wear resistance of polyethylene. Clin Orthop Relat Res. 2003;417:253–62.
13.
Sychterz CJ, Orishimo KF, Engh CA. Sterilization and polyethylene wear: clinical studies to support laboratory data. J Bone Joint Surg Am. 2004;86-A(5):1017–22.
5 Surgical Management of the Young Adult with Hip Pain
Bryan D. Haughom12 and Jared M. Newman11
(11)
Department of Orthopaedic Surgery, Hospital for Special Surgery, 535 E 70th Street, 10021 New York, USA
(12)
Department of Orthopaedic Surgery, Rush University Hospital, 1611 W Harrison Street, Chicago, IL 60612, USA
Take-Home Message
Hip pain encompasses a wide differential diagnosis including neurologic, gynecologic, urologic, gastrointestinal, and musculoskeletal etiologies.
Surgical treatment of hip pain is largely centered upon management of bony deformity, and the pathology can be categorized as primarily acetabular (undercoverage vs. overcoverage) as well as femoral (abnormal version vs. abnormal head-neck offset).
Radiographic evaluation is crucial in determining the fundamental underlying pathology.
Goals of surgery are to create a stable painless hip and to prevent cartilage degeneration.
Background
Hip pain in a young adult or an adult patient may encompass a wide spectrum of disorders, ranging from neurologic, gynecologic, urologic, gastrointestinal, and orthopedic.
Abnormalities of the bony femoroacetabular joint, including both primary femoral-sided and acetabular-sided disease.
Acetabular dysplasia – can be broken into three broad categories:
Overcoverage (i.e., pincer deformity)
Undercoverage (i.e., classic hip dysplasia)
Abnormal acetabular version
Femoral dysplasia – abnormal proximal femoral geometry can include abnormalities of the following:
Femoral neck version
Femoral head-neck offset (i.e., CAM deformity).
Persistent hip pain in a young adult (<50 years of age) is often due to an underlying bony abnormality (femoral or acetabular).
Bony abnormalities of the hip joint may represent an untreated condition from birth/youth, or it may be the sequalae of previously diagnosed and/or treated hip dysplasia.
Underlying bony pathology (i.e., hip dysplasia) is thought to account for a large share of the young adult hip arthritis seen by arthroplasty surgeons.
An increasing effort to diagnose and treat these conditions prior to degenerative changes is encouraged, though definitive proof of a true causative relationship is lacking.
Traditional open, minimally invasive open, and arthroscopic techniques are described to address the myriad of conditions that are associated with adult hip dysplasia.
Etiology
The etiology of bony hip disease is diverse. Primary femoral and acetabular-sided disease has been described; however, often both exist concurrently.
Many disorders have been described as etiologies of dysplasia including developmental hip dysplasia, acetabular retroversion, acetabular protrusio, proximal femoral valgus, Legg-Calvé-Perthes disease, and chronic slipped capital femoral epiphysis to name a few.
More subtle abnormalities including mild acetabular dysplasia, focal acetabular overcoverage, and CAM deformities with time may also lead to early onset hip pain.
Pathophysiology
The fundamental pathology at play in is a mismatch between the sphericity of the femoral head and the congruence of the acetabulum.
Relative undercoverage (i.e., classic dysplasia) may result in a decreased contact area and increased loads over a smaller effective area.
Relative overcoverage, in the case of the acetabulum, or an aspherical femoral head, in the case of a CAM lesion, may lead to impingement of the soft tissue labrum.
In response to the underlying bony abnormalities, muscular, postural, capsular adaptation, and cartilage degeneration may ensue which may lead to pain.
Patients often present with complains of the insidious onset of groin pain (particularly with flexion adduction and internal rotation (FADDIR), and flexion abduction and external rotation (FABER)).
Physical examination may demonstrate decreased or asymmetric hip range of motion (particularly in internal rotation) as well as hip abductor weakness and possibly leg length discrepancy. Provocative tests (i.e., impingement tests such as the FADDIR and FABER tests) may reproduce the pain.
Radiography
Standard radiographs include an anteroposterior (AP) view of pelvis as well as AP and lateral views of the hip. Alternative views such as the Dunn lateral and the false profile view have been advocated as well.
Advanced imaging including CT and MRI scans may provide information about the three-dimensional bony architecture (CT scan) and the capsulolabral structures (MRI scan).
Radiographic measurements include:
Alpha angle
Angle formed between the axis of the femoral neck, the center of the femoral head, and the point at which the femoral head becomes aspherical.
May be measured on all views.
CAM deformity is defined as alpha angle >55°.
Center–edge angle (CEA)
Angle formed between a perpendicular to the axis of the pelvis (i.e., a line connected between the ischial tuberosities) and the lateral margin of the acetabular dome.
Pincer deformity is defined as CEA > 39°.
Classic hip dysplasia is defined as CEA < 20°.
Mild hip dysplasia: 20–25°
Acetabular index (Tonnis angle)
Measured on AP pelvis radiograph.
Angle formed between a horizontal line at the apex of the acetabular tear drop and the lateral edge of the acetabulum.
Abnormal values include >10° and <0°.
Additional radiographic findings
Crossover Sign
Measured on the AP pelvis radiographRelated posts:
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