179 © 2014 Chinese Orthopaedic Association and Wiley Publishing Asia Pty Ltd
REVIEW ARTICLE
How to Interpret Postoperative X-rays after Total Knee Arthroplasty Nishikant Kumar, MS, Chandrashekhar Yadav, MS, Rishi Raj, MS, Sumit Anand, MS All India Institute of Medical Sciences, Orthopaedics, Ansari Nagar Aiims, New Delhi, India
Today, total knee arthroplasty (TKA) is one the most commonly performed surgeries worldwide. The purpose of this article is to review the appearance of normal post-TKA roentgenographs and describe the correct sequence for their interpretation. It is unwise to depend solely on patients’ symptoms when diagnosing TKA complications because serial radiographs can foresee failures well before they manifest clinically. Ideal post-TKA radiographs comprise whole lower extremity anteroposterior and lateral views taken under weight bearing conditions along with a skyline view of the patellofemoral joint. Among other things, weight bearing exposes the true alignment, ligamentous laxity and polyethylene wear. On the basis of follow-up of our TKA cases, we have drawn up a protocol for assessing postoperative X-ray films after TKAs. Following the proposed sequence, surgeon can easily decide how to proceed with follow-up and foresee complications. Careful interpretation of postoperative radiographs after TKA is essential to careful monitoring of patients and implant survival.
Key words: Arthroplasty; Interpretation; Knee; Roentgenography
Introduction raditional teaching says “treat patients and not their radiographs”; however, more recent experience suggests “do not apply this rule to total knee arthroplasty (TKA) patients”. It is unwise to depend solely on patients’ symptoms when diagnosing TKA complications because serial radiographs can foresee failures well before they manifest clinically1. Radiography plays a significant role in TKA in both the immediate postoperative period and during long-term follow-up. However, orthopaedic surgeons are very inept at reading post-TKA radiographs and often fail to perceive the vast amount of information they contain. Provided the surgeon knows what to look at in the radiographs and the optimum sequence to follow, no other radiological investigation is required post-TKA. Ideal post-TKA radiographs comprise whole lower extremity anteroposterior and lateral views taken under weight bearing conditions along with a skyline view of the patellofemoral joint. Among other things, weight bearing exposes the true alignment, ligamentous laxity and polyethylene wear. We propose the following sequence of observation for reading post-TKA radiographs.
T
Implantation good quality radiograph can identify the type of implant used, if not the exact brand. An effort should be made to categorize implants as: (i) posterior cruciate ligament-retaining, posterior cruciate ligament-sacrificing and posterior stabilized; (ii) cemented, uncemented and hybrid; (iii) fixed-bearing and mobilebearing; (iv) varied tibial component; and (v) constrained or unconstrained prosthesis. With posterior stabilized prostheses, the tibial post and femoral cam substitute for the posterior cruciate ligament; however, the post and cam cannot be directly visualized on radiograph. It is the absence of the additional piece of bone that has been removed from the intercondylar notch to accommodate the femoral cam that points towards use of such implants1–4. To differentiate between posterior cruciate ligament retaining and posterior cruciate ligament sacrificing prostheses is much more difficult, requiring radiographs with the appropriate X-ray penetration for outlining the surface of the radiolucent polyethylene insert2–4. This surface is flat in posterior cruciate ligament retaining prostheses (to facilitate
A
Address for correspondence Nishikant Kumar, MS, All India Institute of Medical Sciences, Orthopaedics, Ansari Nagar Aiims, New Delhi, India 110029 Tel: 9560416858; Fax: 06542232055; Email: [email protected] Disclosure: All authors declare no conflicts of interest. Received 9 October 2013; accepted 17 March 2014
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Orthopaedic Surgery 2014;6:179–186 • DOI: 10.1111/os.12123
180 Orthopaedic Surgery Volume 6 · Number 3 · August, 2014
femoral roll-back), whereas it is dished in posterior cruciate ligament sacrificing prostheses2–4. The difference between cemented and uncemented TKA on a radiograph is glaringly obvious: the implant margin appears roughened and beaded in the latter type, whereas an obvious radio-opacity surrounds the more common cemented type1,4,5. When femoral components are inserted without cement and tibial components with cement, this is termed hybrid implantation. One of the most important factors affecting implant survival is the quality of cementation: ideally, there should be at least 2 mm of cement mantle around the implant interdigitating with the cancellous bone6. Fixed-bearing prostheses, which have a polyethylene tibial insert locked within the tibial tray, can be distinguished radiologically from mobile bearing prostheses, which allow translation and rotation of the tibial insert in relation to the tray. The mobile bearing prostheses have metal markers inside the insert to allow assessment of the rotational status of the femorotibial articulation and metallic pins, pegs, studs and control arms in the middle of the articulation or stops or capture rims at the periphery to guide or restrict the mobility of the insert. A variety of tibial component designs, for example, metal backed monobloc designs and modular tibial components are difficult to distinguish radiologically. However, all polyethylene designs stand out clearly because of the absence of radio-opacity on the tibial side. In the presence of bony defects and increased constraint at the femorotibial articula-
Interpretation Postop X-rays of TKA
Fig. 2 Postoperative radiograph of TKR showing MDFA of 95° and MPTA of 85°. The tibial angle here is suboptimal.
tion, long stemmed femurs or tibias are used. Radiographs to evaluate these should show whether the stem is offset, cement was used with the stem and the length of the stem used is adequate. We should not confuse varus-valgus-constraint polyethylene inserts with hinged prostheses because the former have only a metallic pin inside the tibial post, whereas the latter have heavier metal at the tibio-femoral junction. Augments (bone graft and cement) and small pieces of hardware such as screws and the staples used to fix them are visible in particular radiographic views (Fig. 1). Surgical Procedure ostoperative radiography is performed to assess the adequacy of the procedure.
P
Implant Positioning Restoration of the mechanical axis in the coronal plane and adequate orientation in sagittal plane are required to conclude that an implant is properly positioned.
Fig. 1 Long tibial cemented insert (with stem extender) for occult fracture of proximal tibia (the occult fracture is indicated by the arrow).
Mechanical Axis (Anteroposterior View) Ideally, a long anteroposterior (AP) film of the whole lower limb should be available: the mechanical axis drawn on it should be perpendicular to the knee joint line and pass near or through the center of the knee. If only a short AP film is available, the medial distal femoral angle (MDFA) and medial proximal tibial angle (MPTA) can still be calculated; these should normally be 95° and 90°, respectively (Fig. 2), the sum being less than 185° and hence the knee being in 5° of valgus5,7,8. It is important to avoid varus-varus alignment of the
181 Orthopaedic Surgery Volume 6 · Number 3 · August, 2014
Fig. 3 Lateral view showing Grade II notching of anterior femoral cortex.
components7. Short films are inadequate if there are abnormal femoral and tibial curves. The MDFA is calculated on short films by drawing a tangent to the distal femoral condyles and between the femoral anatomical axes. The MPTA is similarly calculated by drawing a tangent across the tibial base plate and between the anatomical axes of the tibia5,8. Sagittal Plane (Lateral View) Alignment of the femoral and tibial components should be assessed in a lateral view. The femoral component should be in a neutral position because excessive extension poses a risk of notching of the anterior femoral cortex whereas excessive flexion inhibits knee extension1,5. Notching is erosion of the anterior femoral cortex to various degrees caused by the femoral component (Fig. 3). Notching of the femur increases stress concentration at the anterior femoral cortex, and thus the chances of supracondylar periprosthetic fracture9. Notching cautions protective manipulation of the knee in postoperative period. Gujarathi et al. have proposed the following four grades of femoral component notching9: Grade 1, violation of the outer table of the anterior femoral cortex; Grade 2, violation of the outer and inner table of the anterior femoral cortex; Grade 3, violation of up to 25% of the medullary canal; and Grade 4, violation of up to 50% of the medullary canal. The tibial component has different posterior slopes depending on the specific prosthesis design; however, in general posterior cruciate ligament-retaining prostheses require more slope than posterior stabilized implants. Most of the current designs aim for a posterior slope of 3–7°. Because the anterior portion of the cut tibia is soft, there is a chance of subsistence of the tibial component. This fact accounts for the
Interpretation Postop X-rays of TKA
normal posterior slope of the tibial component. Precautions should be taken while cutting the tibia because an excessive posterior slope causes flexion instability whereas a smaller or anterior slope leads to tight collateral ligaments and hence decreased knee flexion10. The posterior condylar offset (Fig. 4)10,11 is the maximal thickness of the posterior femoral condyle projected posteriorly to the tangential of the posterior cortex of the femoral shaft and seen on lateral radiographs; it has to be maintained after TKA10,11. Excessive reduction of the posterior condylar offset after TKA is undesirable, leading to loose flexion space and flexion instability. This also causes a lax posterior cruciate ligament, which leads to paradoxical roll-forward of the femur on the tibia during flexion. Normally, the femur rolls backward on the tibia (roll backward phenomenon) during flexion. The posterior condylar offset ratio11 (Fig. 5) is defined as the maximum thickness of the posterior condyle projecting posteriorly to a straight line drawn as the extension of the posterior femoral shaft cortex, divided by the maximal thickness of the posterior condyle projecting posterior to a straight line drawn as the extension of the anterior femoral shaft cortex on a true lateral radiograph of the distal quarter of the femur: its normal value is 0.47 on post-TKA radiographs. Implant Size deal implants mirror the natural anatomy and the margins of the component are flush with the respective cortical surfaces1,5. A little lateral overhang of the femoral component is
I
Fig. 4 Diagrammatic depiction of measurement of posterior condylar offset (A) before surgery and (B) after surgery. A Should be equal to B to prevent mid flexion instability.
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Interpretation Postop X-rays of TKA
Fig. 6 Insall–Salvati ratio (P/LP) and Canton–Deshamps index (AP/ AT) for assessing patellar height.
Fig. 5 Lateral view of postoperative TKR showing restoration of posterior condylar offset.
acceptable. An oversized femoral component leaves a gap between the anterior flange and the anterior femoral cortex, whereas an undersized component leads to anterior notching, provided the component is in neutral rotation in the sagittal plane. A large component decreases knee flexion by overstuffing the patellofemoral joint and creating a tight flexion gap, whereas a small component does not fill the flexion gap adequately, leading to flexion instability12. Small tibial components cause subsidence into the cancellous bone whereas large components cause soft tissue irritation and ligament imbalance, leading to decreased motion. Ideally, there should be no medial overhang of the femoral component or posterior overhang of the tibial component12.
The Caton–Deschamps index is the ratio of the distance between the lower edge of the patellar articular surface and anterosuperior margin of the tibia to the length of the articular surface of the patella. Skyline View This view shows the patellar thickness, which is important in patellar tracking and affects patellofemoral kinematics. Anteroposterior View The ideal position of the patellar component is the medial two-thirds of the knee; a laterally placed patella predisposes to maltracking and lateral subluxation14 (Fig. 7). The orientation of the patella in the coronal plane can be assessed using the Q-angle. This is the angle between a line from the anterior superior iliac spine to the center of the patella and a second line
Patella Lateral View Supero-inferior orientation of patella can be assessed in lateral radiographs using the Insall–Salvati or Canton–Deschamps ratio (Fig. 6)13,14. The Insall–Salvati is the ratio of the greatest length of the patella divided by the length of the patellar tendon. Normal values lie between 0.8–1.2. Values 4; regular follow-up is recommended if it is 5–9 and >10 predicts impending failure. A stable line less than 2 mm is insignificant if it appears within a year and stabilizes therefter15. Aseptic Loosening The two most important radiological predictors of loosening are a progressive zone of radiolucency at the interfaces around the components and a change in the position of components
Fig. 9 Osteolysis around the tibial tray (arrows).
(Fig. 9). Either cemented or uncemented components may develop thin lucencies (2 mm signify loosening. Such aseptic loosening is more common with tibial components and is the most common cause of revision of TKAs1,15,16. Changes in component position very reliably predict loosening. In tibial component loosening, the tibial tray can sink into the tibial plateau, which is referred to as “subsistence”, or the tibial component may shift into a varus position with respect to the long axis of tibia. Likewise, loosened femoral components shift into a flexed position with respect to the long axis of the femur in lateral radiographs; however, this happens less frequently than tibial loosening16. Stress patellar component loosening presents with subtle radiological signs. Septic Loosening Though difficult to distinguish from aseptic loosening, extensive, ill-defined radiolucent zones with or without periosteal reaction favor a diagnosis of septic loosening. Various radionuclide imaging modalities have been employed to differentiate between septic and aseptic loosening. These include fludeoxyglucose positron emission tomography, triple phase bone and indium-111 leucocyte scans17. Although radionuclide imaging plays an important role in the diagnostic workup, none of the modalities available serves as a gold standard technique because each of the radionuclide modalities has its drawbacks and limitations.
Fig. 8 Different cement zones on femoral and tibial sides.
Polyethylene Wear and Osteolysis There are four modes of wear18. Mode I is an articulation between intended bearing surfaces (i.e., between the femoral
184 Orthopaedic Surgery Volume 6 · Number 3 · August, 2014
Fig. 10 Merchant view showing patellar subluxation following TKA.
condyle and tibial insert; mode II is articulation between the primary bearing surface (femoral condyle) and a surface that was never intended to be a bearing surface (metal backing of patellar component); mode III is between intentional bearing surfaces in the presence of third body components (polymethyl methacrylate debris, metallic debris, ceramic debris, bone particles, etc.); and mode IV is articulation between two nonbearing secondary surfaces (back of polyethylene insert and metallic tray). The types of wear include: (i) adhesive wear; (ii) abrasive wear; (iii) third body wear; (iv) volumetric wear; and (v) linear wear. The shortest distance from each femoral condyle to a transverse line through the middle of superior surface of the base plate in AP and lateral views provides an estimate of the remaining insert thickness. It is very important to note that the X-ray beam should be parallel to the tibial base plate when measuring the extent of polyethylene wear, if not, the interpre-
Interpretation Postop X-rays of TKA
tation can be confusing. Polyethylene wear leads not only to decreased joint space and thus loss of ligament balance (leading to instability), but also to subsequent metal-on-metal articulation between the femoral component and tibial base plate over time4. This leads to metallosis within the joint; the metal particles are visualized as radiodensities outlining the suprapatellar recess of the joint capsule18. Such wear debris is implicated in the mechanism of periprosthetic osteolysis and hence aseptic loosening. Because osteolysis can be clinically asymptomatic and radiologically difficult to identify because of the presence of cancellous bone and masking by components, there should be a high index of suspicion when looking for osteolysis in high risk patients4,10,18,19. An active young man who has had an implant for a long time and develops metalon-metal crepitus and excess instability is highly likely to have osteolysis. Also, osteolysis is more common with heat-pressed and carbon-reinforced inserts, titanium femoral components and poor locking between the tibial tray and a modular insert18,19. Instability of Patellofemoral Joint After TKA, the patella should lie in the center of the trochlear groove in merchant views (Fig. 10)14,19. The patellar tilt is the angle formed between the patellar bone–prosthesis interface and a line drawn across the anterior femoral condyles. A patellar tilt >5° shows patellofemoral instability1,14,19. Patellar Fracture Patellar fracture20 following TKA is infrequent (Fig. 11). It may occur for the following reasons12,20: (i) devascularisation of the patella (excessive lateral retinacular release which compromises the superior-lateral geniculate artery; (ii) strong patellar
Fig. 11 Disruption of patellar component with posterior suluxation of patella (high riding patella; chip avulsion from tibial tuberosity).
185 Orthopaedic Surgery Volume 6 · Number 3 · August, 2014
Interpretation Postop X-rays of TKA
Radiographs may not show abnormalities during the acute phase of erythema and swelling. Later radiographs (after 1–2 weeks) show only soft tissue swelling. HO becomes obvious on X-ray films by a mean of 5 weeks and may take 8–14 months to reach maturity. Triple phase bone scanning is the preferred modality for early HO detection21,23,24. Rader et al. proposed a classification of HO that focusses on the functional impairment of patients and is correlated with their radiographic data26: Class 0, No HO; Class I, largest HO 5 cm2 in lateral or AP radiographs in the extensor apparatus or proximal femur; and Class III, largest HO > 5 cm2 at the extensor apparatus and near the femur. Patients with Rader class III HO have severe restriction of knee flexion that warrants surgical intervention. Dislocation of Femorotibial Articulation Is Evident on X-ray
Fig. 12 Postoperative radiograph showing Rader class III HO (arrows).
strains due to component malalignment; and (iii) oversized femoral components. Patients commonly present with anterior knee pain and extensor lag. Mostly patient can be managed conservatively; however, if displacement is >2 mm, operative intervention is indicated19,20. Hetrotropic Ossification Heterotropic ossification (HO) is defined as formation of lamellar bone in soft tissues (Fig. 12)1,21,22. The incidence of HO after TKA appears to be less than after total hip arthoplasty. There appears to be a genetic predisposition to this disorder. Patients at high risk of this include those with preexisting or contralateral HO, hypertrophic osteoarthritis, ankylosing spondylitis, diffuse idiopathic skeletal hyperostosis, infection21–24; and combined deformity greater than 15°. Surgical technique-related factors that may lead to HO23 include splitting of the quadriceps tendon, stripping the soft tissues on the anterior aspect of the femur, knee effusions, notching of the femur, vigorous soft tissue retraction and manipulation after implantation24,25.
Periprosthetic Fracture Fractures can occur on both tibial and femoral sides either intraoperatively or postoperatively23–26. Predisposing factors for postoperative fractures are osteopenia, particle wear leading to osteolysis and component loosening27–29. A very common site is the supracondylar region, for which anterior femoral notching is an additional risk factor. Fractures of the prosthetic condylar and tibial components are much less common. Risk factors for tibial component fracture include component malalignment, improper cementing, severe polyethylene wear and undersizing of the tibial component, leading to its subsistence. These findings are obvious on appropriate radiographs23–29. Conclusions detailed and comprehensive study of radiographs of TKA helps deciphering of various clues both in the immediate postoperative and follow-up periods. Thorough evaluation of radiographs enables decisions on the next best step, which could be just regular follow-up or a surgical intervention. While it is easy to convince a patient with symptoms of failure and obvious loosening of components that reoperation is advisable, an asymptomatic patient with osteolysis and excess wear should be appropriately counselled and made aware of the risk of sudden liner breakage or bone stock depletion if intervention is delayed. Finally, TKA patients require indefinite follow-up with adequate radiographs at each visit.
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References 1. Chiu KY, Cheng HC, Yau WP, Tang WM, Ho HS. Reading radiographs after total knee arthroplasty. J Orthop Surg (Hong Kong), 2010, 14: 25–39. 2. Scuderi GR, Pagnano MW. Review article: the rationale for posterior cruciate substituting total knee arthroplasty. J Orthop Surg (Hong Kong), 2001, 9: 81–88. 3. Barnes CL, Sledge CB. Total knee arthroplasty with posterior cruciate ligament retention designs. In: Insall JN, Windsor RE, Scott WN, et al., eds. Surgery of the Knee, 2nd edn. New York: Churchill, Livingstone, 1993; 815–827.
4. Allen AM, Ward WG, Pope TL Jr. Imaging of the total knee arthroplasty. Radiol Clin North Am, 1995, 33: 289–303. 5. Berquist TH. Imaging of joint replacement procedures. Radiol Clin North Am, 2006, 44: 419–437. 6. Walker PS, Soudry M, Ewald FC, McVickar H. Control of cement penetration in total knee arthroplasty. Clin Orthop Relat Res, 1984, 185: 155–164. 7. Patel DV, Ferris BD, Aichroth PM. Radiological study of alignment after total knee replacement. Short radiographs or long radiographs? Int Orthop, 1991, 15: 209–210.
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8. Stern SH, Insall JN. Posterior stabilized prosthesis. Results after follow-up of nine to twelve years. J Bone Joint Surg Am, 1992, 74: 980–986. 9. Gujarathi N, Putti AB, Abboud RJ, Maclean JG, Espley AJ, Kellett CF. Risk of periprosthetic fracture after anterior femoral notching. Acta Orthop, 2009, 80: 553–556. 10. Weissman BN. Radiographic evaluation of total joint replacement. In: Sledge CB, Ruddy S, Harris ED Jr, eds. Arthritis Surgery. Philadelphia, PA: WB Saunders, 1994; 846–907. 11. Bellemans J, Banks S, Victor J, Vandenneucker H, Moemans A. Fluoroscopic analysis of the kinematics of deep flexion in total knee arthroplasty: the influence of posterior condylar offset. J Bone Joint Surg Br, 2002, 84: 50–53. 12. Miller TT. Imaging of knee arthroplasty. Eur J Radiol, 2005, 54: 164–177. 13. Rogers BA, Thornton Bott P, Cannon SR, Briggs TW. Interobserver variation in the measurement of patellar height after total knee arthroplasty. J Bone Joint Surg Br, 2006, 88: 484–488. 14. Merchant AC, Mercer RL, Jacobsen RH, Cool CR. Roentgenographic analysis of patellofemoral congruence. J Bone Joint Surg Am, 1974, 56: 1391–1396. 15. Schneider R, Hood RW, Ranawat CS. Radiographic evaluation of knee arthroplasty. Orthop Clin North Am, 1982, 13: 225–244. 16. Math KR, Schieder R. Imaging of the painful TKR. In: Scuderi GR, Tria AJ, eds. Surgical Techniques in Total Knee Arthroplasty. New York: Springer-verlag, 2002; 351–367. 17. Palestro CJ, Swyer AJ, Kim CK, Goldsmith SJ. Infected knee prosthesis: diagnosis with In-111 leukocyte, Tc-99m sulfur colloid, and Tc-99m MDP imaging. Radiology, 1991, 179: 645–648. 18. Schmalzried TP, Callaghan JJ. Wear in total hip and knee replacements. J Bone Joint Surg Am, 1999, 81: 115–136.
Interpretation Postop X-rays of TKA
19. Benjamin JB, Lund PJ. Orthopedic devices. In: Hunter TB, Bragg DG, eds. Radiological Guide to Medical Devices and Foreign Bodies. St. Louis, MO: Mosby, 1994; 348–385. 20. Grelsamer RP, Bazos AN, Proctor CS. Radiographic analysis of patellar tilt. J Bone Joint Surg Br, 1993, 75: 822–824. 21. Ortiguera CJ, Berry DJ. Patellar fracture after total knee arthroplasty. J Bone Joint Surg Am, 2002, 84: 532–540. 22. Furia JP, Pellegrini VD Jr. Heterotopic ossification following primary total knee arthroplasty. J Arthroplasty, 1995, 10: 413–419. 23. Pham J, Kumar R. Heterotopic ossification after total knee arthroplasty. Am J Orthop, 1997, 26: 141–143. 24. Platzer P, Schuster R, Aldrian S, et al. Management and outcome of periprosthetic fracture after total knee arthroplasty. J Trauma, 2010, 68: 1464–1470. 25. Dalury DF, Jiranek WA. The incidence of heterotopic ossification after total knee arthroplasty. J Arthroplasty, 2004, 19: 447–452. 26. Iorio R, Healy WL. Heterotopic ossification after hip and knee arthroplasty: risk factors, prevention, and treatment. J Am Acad Orthop Surg, 2002, 10: 409–416. 27. Rader CP, Barthel T, Haase M, Scheidler M, Euler J. Heterotopic ossification after total knee arthroplasty. 54/615 cases after 1–6 years’ follow-up. Acta Orthop Scand, 1997, 68: 46–50. 28. Jansen JA, Smit F, Arias-Bouda LMP. The role of nuclear medicine techniques in differentiation between septic and aseptic loosening of total hip and knee arthroplasty. Tijdschr Nucl Geneesk, 2012, 34: 988–994. 29. Math KR, Zaidi SF, Petchprapa C, Harwin SF. Imaging of total knee arthroplasty. Semin Musculoskelet Radiol, 2006, 10: 47–63.
REVIEW ARTICLE
How to Interpret Postoperative X-rays after Total Knee Arthroplasty Nishikant Kumar, MS, Chandrashekhar Yadav, MS, Rishi Raj, MS, Sumit Anand, MS All India Institute of Medical Sciences, Orthopaedics, Ansari Nagar Aiims, New Delhi, India
Today, total knee arthroplasty (TKA) is one the most commonly performed surgeries worldwide. The purpose of this article is to review the appearance of normal post-TKA roentgenographs and describe the correct sequence for their interpretation. It is unwise to depend solely on patients’ symptoms when diagnosing TKA complications because serial radiographs can foresee failures well before they manifest clinically. Ideal post-TKA radiographs comprise whole lower extremity anteroposterior and lateral views taken under weight bearing conditions along with a skyline view of the patellofemoral joint. Among other things, weight bearing exposes the true alignment, ligamentous laxity and polyethylene wear. On the basis of follow-up of our TKA cases, we have drawn up a protocol for assessing postoperative X-ray films after TKAs. Following the proposed sequence, surgeon can easily decide how to proceed with follow-up and foresee complications. Careful interpretation of postoperative radiographs after TKA is essential to careful monitoring of patients and implant survival.
Key words: Arthroplasty; Interpretation; Knee; Roentgenography
Introduction raditional teaching says “treat patients and not their radiographs”; however, more recent experience suggests “do not apply this rule to total knee arthroplasty (TKA) patients”. It is unwise to depend solely on patients’ symptoms when diagnosing TKA complications because serial radiographs can foresee failures well before they manifest clinically1. Radiography plays a significant role in TKA in both the immediate postoperative period and during long-term follow-up. However, orthopaedic surgeons are very inept at reading post-TKA radiographs and often fail to perceive the vast amount of information they contain. Provided the surgeon knows what to look at in the radiographs and the optimum sequence to follow, no other radiological investigation is required post-TKA. Ideal post-TKA radiographs comprise whole lower extremity anteroposterior and lateral views taken under weight bearing conditions along with a skyline view of the patellofemoral joint. Among other things, weight bearing exposes the true alignment, ligamentous laxity and polyethylene wear. We propose the following sequence of observation for reading post-TKA radiographs.
T
Implantation good quality radiograph can identify the type of implant used, if not the exact brand. An effort should be made to categorize implants as: (i) posterior cruciate ligament-retaining, posterior cruciate ligament-sacrificing and posterior stabilized; (ii) cemented, uncemented and hybrid; (iii) fixed-bearing and mobilebearing; (iv) varied tibial component; and (v) constrained or unconstrained prosthesis. With posterior stabilized prostheses, the tibial post and femoral cam substitute for the posterior cruciate ligament; however, the post and cam cannot be directly visualized on radiograph. It is the absence of the additional piece of bone that has been removed from the intercondylar notch to accommodate the femoral cam that points towards use of such implants1–4. To differentiate between posterior cruciate ligament retaining and posterior cruciate ligament sacrificing prostheses is much more difficult, requiring radiographs with the appropriate X-ray penetration for outlining the surface of the radiolucent polyethylene insert2–4. This surface is flat in posterior cruciate ligament retaining prostheses (to facilitate
A
Address for correspondence Nishikant Kumar, MS, All India Institute of Medical Sciences, Orthopaedics, Ansari Nagar Aiims, New Delhi, India 110029 Tel: 9560416858; Fax: 06542232055; Email: [email protected] Disclosure: All authors declare no conflicts of interest. Received 9 October 2013; accepted 17 March 2014
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Orthopaedic Surgery 2014;6:179–186 • DOI: 10.1111/os.12123
180 Orthopaedic Surgery Volume 6 · Number 3 · August, 2014
femoral roll-back), whereas it is dished in posterior cruciate ligament sacrificing prostheses2–4. The difference between cemented and uncemented TKA on a radiograph is glaringly obvious: the implant margin appears roughened and beaded in the latter type, whereas an obvious radio-opacity surrounds the more common cemented type1,4,5. When femoral components are inserted without cement and tibial components with cement, this is termed hybrid implantation. One of the most important factors affecting implant survival is the quality of cementation: ideally, there should be at least 2 mm of cement mantle around the implant interdigitating with the cancellous bone6. Fixed-bearing prostheses, which have a polyethylene tibial insert locked within the tibial tray, can be distinguished radiologically from mobile bearing prostheses, which allow translation and rotation of the tibial insert in relation to the tray. The mobile bearing prostheses have metal markers inside the insert to allow assessment of the rotational status of the femorotibial articulation and metallic pins, pegs, studs and control arms in the middle of the articulation or stops or capture rims at the periphery to guide or restrict the mobility of the insert. A variety of tibial component designs, for example, metal backed monobloc designs and modular tibial components are difficult to distinguish radiologically. However, all polyethylene designs stand out clearly because of the absence of radio-opacity on the tibial side. In the presence of bony defects and increased constraint at the femorotibial articula-
Interpretation Postop X-rays of TKA
Fig. 2 Postoperative radiograph of TKR showing MDFA of 95° and MPTA of 85°. The tibial angle here is suboptimal.
tion, long stemmed femurs or tibias are used. Radiographs to evaluate these should show whether the stem is offset, cement was used with the stem and the length of the stem used is adequate. We should not confuse varus-valgus-constraint polyethylene inserts with hinged prostheses because the former have only a metallic pin inside the tibial post, whereas the latter have heavier metal at the tibio-femoral junction. Augments (bone graft and cement) and small pieces of hardware such as screws and the staples used to fix them are visible in particular radiographic views (Fig. 1). Surgical Procedure ostoperative radiography is performed to assess the adequacy of the procedure.
P
Implant Positioning Restoration of the mechanical axis in the coronal plane and adequate orientation in sagittal plane are required to conclude that an implant is properly positioned.
Fig. 1 Long tibial cemented insert (with stem extender) for occult fracture of proximal tibia (the occult fracture is indicated by the arrow).
Mechanical Axis (Anteroposterior View) Ideally, a long anteroposterior (AP) film of the whole lower limb should be available: the mechanical axis drawn on it should be perpendicular to the knee joint line and pass near or through the center of the knee. If only a short AP film is available, the medial distal femoral angle (MDFA) and medial proximal tibial angle (MPTA) can still be calculated; these should normally be 95° and 90°, respectively (Fig. 2), the sum being less than 185° and hence the knee being in 5° of valgus5,7,8. It is important to avoid varus-varus alignment of the
181 Orthopaedic Surgery Volume 6 · Number 3 · August, 2014
Fig. 3 Lateral view showing Grade II notching of anterior femoral cortex.
components7. Short films are inadequate if there are abnormal femoral and tibial curves. The MDFA is calculated on short films by drawing a tangent to the distal femoral condyles and between the femoral anatomical axes. The MPTA is similarly calculated by drawing a tangent across the tibial base plate and between the anatomical axes of the tibia5,8. Sagittal Plane (Lateral View) Alignment of the femoral and tibial components should be assessed in a lateral view. The femoral component should be in a neutral position because excessive extension poses a risk of notching of the anterior femoral cortex whereas excessive flexion inhibits knee extension1,5. Notching is erosion of the anterior femoral cortex to various degrees caused by the femoral component (Fig. 3). Notching of the femur increases stress concentration at the anterior femoral cortex, and thus the chances of supracondylar periprosthetic fracture9. Notching cautions protective manipulation of the knee in postoperative period. Gujarathi et al. have proposed the following four grades of femoral component notching9: Grade 1, violation of the outer table of the anterior femoral cortex; Grade 2, violation of the outer and inner table of the anterior femoral cortex; Grade 3, violation of up to 25% of the medullary canal; and Grade 4, violation of up to 50% of the medullary canal. The tibial component has different posterior slopes depending on the specific prosthesis design; however, in general posterior cruciate ligament-retaining prostheses require more slope than posterior stabilized implants. Most of the current designs aim for a posterior slope of 3–7°. Because the anterior portion of the cut tibia is soft, there is a chance of subsistence of the tibial component. This fact accounts for the
Interpretation Postop X-rays of TKA
normal posterior slope of the tibial component. Precautions should be taken while cutting the tibia because an excessive posterior slope causes flexion instability whereas a smaller or anterior slope leads to tight collateral ligaments and hence decreased knee flexion10. The posterior condylar offset (Fig. 4)10,11 is the maximal thickness of the posterior femoral condyle projected posteriorly to the tangential of the posterior cortex of the femoral shaft and seen on lateral radiographs; it has to be maintained after TKA10,11. Excessive reduction of the posterior condylar offset after TKA is undesirable, leading to loose flexion space and flexion instability. This also causes a lax posterior cruciate ligament, which leads to paradoxical roll-forward of the femur on the tibia during flexion. Normally, the femur rolls backward on the tibia (roll backward phenomenon) during flexion. The posterior condylar offset ratio11 (Fig. 5) is defined as the maximum thickness of the posterior condyle projecting posteriorly to a straight line drawn as the extension of the posterior femoral shaft cortex, divided by the maximal thickness of the posterior condyle projecting posterior to a straight line drawn as the extension of the anterior femoral shaft cortex on a true lateral radiograph of the distal quarter of the femur: its normal value is 0.47 on post-TKA radiographs. Implant Size deal implants mirror the natural anatomy and the margins of the component are flush with the respective cortical surfaces1,5. A little lateral overhang of the femoral component is
I
Fig. 4 Diagrammatic depiction of measurement of posterior condylar offset (A) before surgery and (B) after surgery. A Should be equal to B to prevent mid flexion instability.
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Fig. 6 Insall–Salvati ratio (P/LP) and Canton–Deshamps index (AP/ AT) for assessing patellar height.
Fig. 5 Lateral view of postoperative TKR showing restoration of posterior condylar offset.
acceptable. An oversized femoral component leaves a gap between the anterior flange and the anterior femoral cortex, whereas an undersized component leads to anterior notching, provided the component is in neutral rotation in the sagittal plane. A large component decreases knee flexion by overstuffing the patellofemoral joint and creating a tight flexion gap, whereas a small component does not fill the flexion gap adequately, leading to flexion instability12. Small tibial components cause subsidence into the cancellous bone whereas large components cause soft tissue irritation and ligament imbalance, leading to decreased motion. Ideally, there should be no medial overhang of the femoral component or posterior overhang of the tibial component12.
The Caton–Deschamps index is the ratio of the distance between the lower edge of the patellar articular surface and anterosuperior margin of the tibia to the length of the articular surface of the patella. Skyline View This view shows the patellar thickness, which is important in patellar tracking and affects patellofemoral kinematics. Anteroposterior View The ideal position of the patellar component is the medial two-thirds of the knee; a laterally placed patella predisposes to maltracking and lateral subluxation14 (Fig. 7). The orientation of the patella in the coronal plane can be assessed using the Q-angle. This is the angle between a line from the anterior superior iliac spine to the center of the patella and a second line
Patella Lateral View Supero-inferior orientation of patella can be assessed in lateral radiographs using the Insall–Salvati or Canton–Deschamps ratio (Fig. 6)13,14. The Insall–Salvati is the ratio of the greatest length of the patella divided by the length of the patellar tendon. Normal values lie between 0.8–1.2. Values 4; regular follow-up is recommended if it is 5–9 and >10 predicts impending failure. A stable line less than 2 mm is insignificant if it appears within a year and stabilizes therefter15. Aseptic Loosening The two most important radiological predictors of loosening are a progressive zone of radiolucency at the interfaces around the components and a change in the position of components
Fig. 9 Osteolysis around the tibial tray (arrows).
(Fig. 9). Either cemented or uncemented components may develop thin lucencies (2 mm signify loosening. Such aseptic loosening is more common with tibial components and is the most common cause of revision of TKAs1,15,16. Changes in component position very reliably predict loosening. In tibial component loosening, the tibial tray can sink into the tibial plateau, which is referred to as “subsistence”, or the tibial component may shift into a varus position with respect to the long axis of tibia. Likewise, loosened femoral components shift into a flexed position with respect to the long axis of the femur in lateral radiographs; however, this happens less frequently than tibial loosening16. Stress patellar component loosening presents with subtle radiological signs. Septic Loosening Though difficult to distinguish from aseptic loosening, extensive, ill-defined radiolucent zones with or without periosteal reaction favor a diagnosis of septic loosening. Various radionuclide imaging modalities have been employed to differentiate between septic and aseptic loosening. These include fludeoxyglucose positron emission tomography, triple phase bone and indium-111 leucocyte scans17. Although radionuclide imaging plays an important role in the diagnostic workup, none of the modalities available serves as a gold standard technique because each of the radionuclide modalities has its drawbacks and limitations.
Fig. 8 Different cement zones on femoral and tibial sides.
Polyethylene Wear and Osteolysis There are four modes of wear18. Mode I is an articulation between intended bearing surfaces (i.e., between the femoral
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Fig. 10 Merchant view showing patellar subluxation following TKA.
condyle and tibial insert; mode II is articulation between the primary bearing surface (femoral condyle) and a surface that was never intended to be a bearing surface (metal backing of patellar component); mode III is between intentional bearing surfaces in the presence of third body components (polymethyl methacrylate debris, metallic debris, ceramic debris, bone particles, etc.); and mode IV is articulation between two nonbearing secondary surfaces (back of polyethylene insert and metallic tray). The types of wear include: (i) adhesive wear; (ii) abrasive wear; (iii) third body wear; (iv) volumetric wear; and (v) linear wear. The shortest distance from each femoral condyle to a transverse line through the middle of superior surface of the base plate in AP and lateral views provides an estimate of the remaining insert thickness. It is very important to note that the X-ray beam should be parallel to the tibial base plate when measuring the extent of polyethylene wear, if not, the interpre-
Interpretation Postop X-rays of TKA
tation can be confusing. Polyethylene wear leads not only to decreased joint space and thus loss of ligament balance (leading to instability), but also to subsequent metal-on-metal articulation between the femoral component and tibial base plate over time4. This leads to metallosis within the joint; the metal particles are visualized as radiodensities outlining the suprapatellar recess of the joint capsule18. Such wear debris is implicated in the mechanism of periprosthetic osteolysis and hence aseptic loosening. Because osteolysis can be clinically asymptomatic and radiologically difficult to identify because of the presence of cancellous bone and masking by components, there should be a high index of suspicion when looking for osteolysis in high risk patients4,10,18,19. An active young man who has had an implant for a long time and develops metalon-metal crepitus and excess instability is highly likely to have osteolysis. Also, osteolysis is more common with heat-pressed and carbon-reinforced inserts, titanium femoral components and poor locking between the tibial tray and a modular insert18,19. Instability of Patellofemoral Joint After TKA, the patella should lie in the center of the trochlear groove in merchant views (Fig. 10)14,19. The patellar tilt is the angle formed between the patellar bone–prosthesis interface and a line drawn across the anterior femoral condyles. A patellar tilt >5° shows patellofemoral instability1,14,19. Patellar Fracture Patellar fracture20 following TKA is infrequent (Fig. 11). It may occur for the following reasons12,20: (i) devascularisation of the patella (excessive lateral retinacular release which compromises the superior-lateral geniculate artery; (ii) strong patellar
Fig. 11 Disruption of patellar component with posterior suluxation of patella (high riding patella; chip avulsion from tibial tuberosity).
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Radiographs may not show abnormalities during the acute phase of erythema and swelling. Later radiographs (after 1–2 weeks) show only soft tissue swelling. HO becomes obvious on X-ray films by a mean of 5 weeks and may take 8–14 months to reach maturity. Triple phase bone scanning is the preferred modality for early HO detection21,23,24. Rader et al. proposed a classification of HO that focusses on the functional impairment of patients and is correlated with their radiographic data26: Class 0, No HO; Class I, largest HO 5 cm2 in lateral or AP radiographs in the extensor apparatus or proximal femur; and Class III, largest HO > 5 cm2 at the extensor apparatus and near the femur. Patients with Rader class III HO have severe restriction of knee flexion that warrants surgical intervention. Dislocation of Femorotibial Articulation Is Evident on X-ray
Fig. 12 Postoperative radiograph showing Rader class III HO (arrows).
strains due to component malalignment; and (iii) oversized femoral components. Patients commonly present with anterior knee pain and extensor lag. Mostly patient can be managed conservatively; however, if displacement is >2 mm, operative intervention is indicated19,20. Hetrotropic Ossification Heterotropic ossification (HO) is defined as formation of lamellar bone in soft tissues (Fig. 12)1,21,22. The incidence of HO after TKA appears to be less than after total hip arthoplasty. There appears to be a genetic predisposition to this disorder. Patients at high risk of this include those with preexisting or contralateral HO, hypertrophic osteoarthritis, ankylosing spondylitis, diffuse idiopathic skeletal hyperostosis, infection21–24; and combined deformity greater than 15°. Surgical technique-related factors that may lead to HO23 include splitting of the quadriceps tendon, stripping the soft tissues on the anterior aspect of the femur, knee effusions, notching of the femur, vigorous soft tissue retraction and manipulation after implantation24,25.
Periprosthetic Fracture Fractures can occur on both tibial and femoral sides either intraoperatively or postoperatively23–26. Predisposing factors for postoperative fractures are osteopenia, particle wear leading to osteolysis and component loosening27–29. A very common site is the supracondylar region, for which anterior femoral notching is an additional risk factor. Fractures of the prosthetic condylar and tibial components are much less common. Risk factors for tibial component fracture include component malalignment, improper cementing, severe polyethylene wear and undersizing of the tibial component, leading to its subsistence. These findings are obvious on appropriate radiographs23–29. Conclusions detailed and comprehensive study of radiographs of TKA helps deciphering of various clues both in the immediate postoperative and follow-up periods. Thorough evaluation of radiographs enables decisions on the next best step, which could be just regular follow-up or a surgical intervention. While it is easy to convince a patient with symptoms of failure and obvious loosening of components that reoperation is advisable, an asymptomatic patient with osteolysis and excess wear should be appropriately counselled and made aware of the risk of sudden liner breakage or bone stock depletion if intervention is delayed. Finally, TKA patients require indefinite follow-up with adequate radiographs at each visit.
A
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