Chapter 39 What Is the Best Treatment for Simple Bone Cysts?
Simple bone cysts (SBCs) are benign bone tumors with a thin, cystlike lining and a fluid-filled cavity.1,2 They occur most commonly in the metaphyses of long bones in skeletally immature patients, although they have been reported in almost every bone of the skeleton.3 The proximal humerus and femur are the most common locations, with 50% to 80% humeral and 20% to 30% femoral in several large studies.4–6 Many cysts heal near skeletal maturity, thereby explaining the rarity in adults, but several large series have patients in their 40s and 50s.4,7, 8 Male-to-female ratios are varied, but two to three times greater in male than female individuals in most large series.4–69 SBCs are one of the most common bone tumors in children, representing 3% of all bone lesion biopsies.10–12 Many more lesions, however, have classic radiographic appearance and are never biopsied. The lesions, although characteristically metaphyseal and central, may rarely cross the physis.13–16 Most SBCs are centrally located and thought to originate from or near a physis. Various theories of cyst origin include physeal aberrations, venous obstruction, and synovial entrapment, or “local disturbance of bone growth and development.”2,7,17–20 SBCs were also called unicameral or solitary cysts. Although normally solitary, multiple concomitant cysts have been reported.21 Monolocular, or unicameral, is the common initial presentation, but fracture or treatment commonly results in septation and multiloculation. Rarely, SBC presents with more extensive ossification, confusing the diagnosis.22,23 Extension into the diaphysis is commonly reported and may represent a large active lesion that extends from the physis to mid-diaphysis, or a small latent diaphyseal cyst that has grown away from the physis.24,25 Cysts are radiolucent because of the fluid content, which is serous in nature but has some unique properties such as greater prostaglandin levels.18,26–30 This fluid under pressure could cause the endosteal erosion and bone expansion that is characteristic of these lesions.31 This expansion may also be due to bone weakness and the body’s attempt to preserve strength by increasing bone diameter.32,33 Cysts that occupy greater then 85% of the bone diameter are associated with increased risk for fracture.34 This is supported by the fact that healing cysts tend to “tubulate”, or narrow, and fragile cysts at risk for fracture are more expanded.35 Fracture in this weakened bone is common and may produce the classic “fallen-fragment” sign.36–38 This small fleck of bone fractures and settles in the cystic content. Fracture initiates a healing response and may cause opacification and septation with the cyst.39 Once the aggressive phase of fracture healing passes, however, the cyst, which appears to be healing, may again become more radiolucent and at risk for future fracture. Garceau and Gregory,40 in 1954, reported 15% cyst healing after fracture, but the true natural history of cyst healing has not been reported because of frequent and varied interventions and treatments. Lesions are reported to recur up to 4 years after initial healing.41 A natural history study was attempted by Neer and researchers16 but abandoned because of frequent fractures. Fractures may result in limb shortening and angular deformity. Physeal involvement may also cause the limb length inequality and angular deformity. Perceived risk for fracture prevents many children from participation in physical activities until cyst resolution. This can be disruptive to childhood for extended periods and limit activities.11,12
Bone cysts were first recognized by Virchow in 1891 and delineated more clearly in a case series by Bloodgood in 1910.42 Since these early times, numerous and various treatments have been proposed and combined. This chapter references more than 130 articles with greater than 20 treatment variations. The fact that so many variations and combinations of treatment for one bone lesion exists suggests the failure of any single current treatment for complete and permanent healing. Most articles are Level IV evidence and represent either single-center or multicenter case experiences with a single treatment. A few articles are comparison studies of two treatments and represent Level III evidence, but only one Level I evidence article is currently available. Another difficulty related to evaluation of cyst treatment is there is no universal system for defining a healed cyst. Neer proposed a classification that has been modified by many, but no study has outlined definitively what constitutes a healed cyst5,8,11,43–49 (Table 39-1). Most practitioners agree that even if a small cyst is present, as long as it does not pose a significant fracture risk and is not increasing in size, then observation is reasonable.4 Recent studies, however, have suggested that plain radiographs may be an inadequate measure of fracture risk and suggest computed tomographic (CT) scan as an alternative measure of fracture risk.32 Magnetic resonance imaging (MRI) study is also promising for assessing the load-carrying capacity of bones with osteolytic lesions.50 With poor outcome tools used for measurement of SBC healing, the comparison of treatment is arbitrary and difficult. Without having an accurate comparison of cysts before treatment, stratification for randomization is also arbitrary. Cysts are known to begin healing and then recur more than 2 to 4 years after healing.41,51 This may represent reactivation of a smaller remaining cystic area or recurrence of a completely healed or resected cyst.52 Capanna and coauthors8 described 12 of 90 patients treated with steroid injection whose cysts recurred after initial consolidation. Most studies have less than 2-year minimum follow-up and, therefore, show early promise for cyst healing, but later extensive follow-up demonstrates recurrence or persistence, or both, of these lesions.4,6, 41 Few articles evaluate the angular deformity and limb-length inequality that results from cysts.53–55 Some of this may be because of the presence of the cyst near the physis, and others may represent physeal damage secondary to treatment. Few studies evaluate whether patients have persistent pain or functional difficulties; however, one recent study does attempt to incorporate functional outcomes using the Activity Scale for Kids (ASK) and pain assessment using the OUCHER scale in a randomized prospective study of two treatments.49,56
TABLE 39-1 Summary of Criteria for Healing
| Graham (1951)43 | |
| Neer et al. (new) (1966)4 | Proposed classification: |
| Incomplete obliteration of the cyst after operation appears to be of little clinical significance, provided there is good bone strength | |
| Spence et al. (new) (1969)44 | |
| Baker (1970)45 | |
| Capanna et al. (modification of Neer Criteria) (1982)8 |
• Healed with residual: cyst consolidated with bone and the cortical margins thickened, but still small residual areas of osteolysis within cyst
|
| Chigara et al. (1983)17 | |
| Oppenheim and Galleno (1984)11 | |
| Lokiec et al. (1996)114 | Features of healing: |
| Weintroub (1989)104 | |
| Hashemi-Nejad and Cole (modification of Neer criteria; Reverse of Neer) (1997)47 | |
| Grade 1–2 = unsatisfactory healing | |
| Grade 3–4 = satisfactory healing | |
| Killian (modification of Neer criteria) (1998)48 | |
| Yandow et al. (1998)49 | Substantial healing |
| Partial healing | |
| Failure to respond | |
| Chang et al. (2002)6 |
Healed: cyst filled by formation of new bone with or without small static, radiolucent area(s) less than 1 cm in size
Healing with defect: static, radiolucent area(s) less than 50% of the diameter of bone with enough cortical thickness to prevent fracture
|
| Wright (modification of Neer and Cole criteria) (2008)56 |
Definition of healing: One of the single greatest difficulties in comparison of treatments and natural history is the definition of healing of simple bone cysts.
FACTORS THAT AFFECT CYST HEALING
Many factors related to the location of the cyst and its host have been proposed to affect the rate of healing.57,58 Most of the series that discuss cyst treatment qualify the results by separating these various factors, which are discussed later. This, however, does not separate the treatment groups before initiation of treatment and, therefore, does not provide equivalent groups for study purposes. More randomized prospective trials will be needed to clarify factors that may affect healing and then to compare headto-head the various treatments.
Lesion Size
Cysts can vary from less than 1 cm2 on two orthogonal views to large complex lesions extending from physis to mid-diaphysis, or more than 50% the length of the bone.49 Cyst index based on size and geography of the lesion was described by Kaelin and McEwen60 as a predictor for fracture risk, but a recent article suggests poor reliability for this technique as a predictor of fracture.59 Others have demonstrated that larger cyst areas correlate with poorer healing.8,56 Capanna and coauthors8 defined small cysts as less than 24 cm2. Spence and coworkers,44 in 1969, in a large multicenter review, showed 80% healing of small lesions, 49% of medium lesions, and 53% of large cysts. This makes excellent sense because larger lesions theoretically have been present for a longer time and may be recalcitrant to the body’s attempts at healing.
Patient Age and Sex
Cysts are uniformly reported in all series reviewed to occur more frequently in male than female individuals. The most common male-to-female ratio is 3:1.9,44 The only exception is 1 smaller series by Dorman121 in 2005 with a 1:1 ratio. The variation in number of male and female patients may affect the healing rates, as Spence suggested in his large multicenter series review of 177 cases. Cysts occur in all age groups. Originally, patients younger than 10 years were considered to have more active cysts.31 Capanna and coauthors,8 in 1982, found less recurrence of cysts after steroid treatment in patients 0 to 5 years of age and poorer healing in those older than 6. Spence and coworkers5 confirm in his large review the adverse effect of younger age on healing. Spence and coworkers also report that female patients had healed cysts in 77% of cases, whereas only 48% of male patients had healed cysts in his review (P < 0.001).44 Case series by Chigira46, Garceau40, Neer4, and Capanna8 include patients with cysts at older than 50 years. The overall lower frequency of this lesion in adulthood in many articles, however, helps us to understand that physeal closure and skeletal maturity does, in some unknown way, promote cyst healing.61 This is important because many of the reviewed articles vary in average age from 6.9 to 17.3 years.62,63 This variation of age may be the cause of variations in healing rates.
Proximity to Physis
Cysts were categorized as active and latent originally by Jaffe and Lichtenstein7 in 1942, based on proximity to the physis. Lesions less than 1 cm from the physis are considered active. Variation of healing response of active verses latent cysts exists in numerous reported series since that time.40,45, 64 Neer, however, in a large series of 175 cases, found no difference in healing of active verses latent cysts.4 Spence, in contrast, showed healing in 35% of active cysts and 67% of latent cysts.5 The data are inconsistent regarding whether location of cyst affects healing. Many articles even fail to evaluate this before treatment.65,66 No trials of similar treatment in randomized groups of active verses latent cysts have been reported to clearly compare location as a unique factor related to healing.
Cyst Fluid and Venous Outflow.
An early study of cyst fluid was conducted by Cohen in 1960.18 It was found to be either serous or bloody, but no correlation to healing was made. Needle perforation and measurement of manometric pressure was elucidated by Neer and coworkers4 in 1966. Active cysts had pressures of 30 cm of H2O and pulsated with Valsalva maneuver. Enneking,31 using this technique, also performed cystograms with injections of radiopaque contrast material attempting to correlate active lesions with rapid outflow of contrast into the venous circulation, and puddling and slow outflow with latent cysts. These are Level IV, observational, and Level V expert opinion articles. Draining or venting the cystic cavity is described in various articles with K-wires, cannulated screws, and intramedullary rods, and these articles are reported and level of evidence reviewed in this summary separately.
Bone Location
SBCs are reported in almost every bone; however, jaw lesions have a different histologic appearance and may be a separate entity. For the purposes of this chapter, these are not included. Greater than 90% of cysts occur in long bones. The calcaneus represents an unusual location, and treatment series for this bone are reported separately in this chapter. The proximal humerus is the most common location. In 195 cases reviewed by Mirra,10 44% were proximal humerus, and the second most common condition was proximal femur with 26% of cases. Hands are a rare location for SBCs. Neer’s series4 showed the femur as the most common location for cyst in adulthood, but fractures of adult cyst were also rarer. Healing rates for treatment of femoral cysts may be higher because of the weight-bearing status of the bone initiating a healing response because of added load. This weight-bearing bone, however, makes this location more at risk for fracture. Angular deformities are reported in numerous studies and may lead to the benefit of intramedullary devices for load sharing or external support during healing.65,67 Location also refers to the location of the cyst within the bone. Metaphysis is the most common initial presentation. Although called simple cysts, their geography and variability is extensive. Cross-sectional imaging (MRI and CT scan) is adding to a more detailed understanding of these lesions.68,69
Length of Follow-Up
Another of the significant difficulties in comparing healing rates of various treatments is the length of post-treatment follow-up. Follow-up varies from 6 months to 20 years in one series.70 Enneking31 states that fracture or intervention to the cyst initially stimulates an aggressive biologic fracture healing response. This promotes early bone formation and opacification of the cyst and “healing.” Once this process becomes more quiescent, the tumor’s biology may again become more active and cause “recurrence” or “visible reappearance” of the lesion.71 Many series report recurrence more than 2 years after treatment in lesions that initially “healed.”5,72 Docquier and Delloye73 had a recurrence at 89 months (7.4 years) after treatment. Therefore, uniform, longer length of follow-up may ruin early positive results. Lokiec and Wientroub12 report 100% healing with bone marrow injection in his preliminary report, but follow-up in some patients was only 12 months. Neer’s report carries such long-term value because of his insistence of a minimum 2-year follow-up period.
Capanna and coauthors describe an interesting phenomenon not studied by others since his articles in 1982.8 Recurrences occurred at varying rates based on healing patterns:
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